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'''Oxygen''' is a chemical element with the symbol&nbsp;'''O''' and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and a potent oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is the most abundant element in Earth's crust, and the third-most abundant element in the universe after hydrogen and helium.

At standard temperature and pressure, two oxygen atoms will bind covalently to form dioxygen, a colorless and odorless diatomic gas with the chemical formula {{chem|O|2}}. Dioxygen gas currently constitutes 20.95% molar fraction of the Earth's atmosphere, though this has changed considerably over long periods of time in Earth's history. Oxygen makes up almost half of the Earth's crust in the form of various oxides such as water, carbon dioxide, iron oxides and silicates.<ref name="Atkins7th">Atkins, P.; Jones, L.; Laverman, L. (2016).''Chemical Principles'', 7th edition. Freeman. {{ISBN|978-1-4641-8395-9}}</ref>

All eukaryotic organisms, including plants, animals, fungi, algae and most protists, need oxygen for cellular respiration, which extracts chemical energy by the reaction of oxygen with organic molecules derived from food and releases carbon dioxide as a waste product. In aquatic animals, dissolved oxygen in water is absorbed by specialized respiratory organs called gills, through the skin or via the gut; in terrestrial animals such as tetrapods, oxygen in air is actively taken into the body via specialized organs known as lungs, where gas exchange takes place to diffuse oxygen into the blood and carbon dioxide out, and the body's circulatory system then transports the oxygen to other tissues where cellular respiration takes place.<ref>{{cite book|last1=Hall|first1=John|title=Guyton and Hall textbook of medical physiology|date=2011|publisher=Saunders/Elsevier|location=Philadelphia, Pa.|isbn=978-1-4160-4574-8|page=5|edition=12th}}</ref><ref name="Pocock2">{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology : the basis of medicine|date=2006|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-856878-0|page=311|edition=3rd}}</ref> However in insects, the most successful and biodiverse terrestrial clade, oxygen is directly conducted to the internal tissues via a deep network of airways.

Many major classes of organic molecules in living organisms contain oxygen atoms, such as proteins, nucleic acids, carbohydrates and fats, as do the major constituent inorganic compounds of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen in Earth's atmosphere is produced by biotic photosynthesis, in which photon energy in sunlight is captured by chlorophyll to split water molecules and then react with carbon dioxide to produce carbohydrates and oxygen is released as a byproduct. Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic activities of autotrophs such as cyanobacteria, chloroplast-bearing algae and plants. A much rarer triatomic allotrope of oxygen, ozone ({{chem|O|3}}), strongly absorbs the UVB and UVC wavelengths and forms a protective ozone layer at the lower stratosphere, which shields the biosphere from ionizing ultraviolet radiation. However, ozone present at the surface is a corrosive byproduct of smog and thus an air pollutant.

Oxygen was isolated by Michael Sendivogius before 1604, but it is commonly believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774. Priority is often given for Priestley because his work was published first. Priestley, however, called oxygen "dephlogisticated air", and did not recognize it as a chemical element. The name ''oxygen'' was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and correctly characterized the role it plays in combustion.

Common industrial uses of oxygen include production of steel, plastics and textiles, brazing, welding and cutting of steels and other metals, rocket propellant, oxygen therapy, and life support systems in aircraft, submarines, spaceflight and diving.

==History of study==
===Early experiments===
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century&nbsp;BCE Greek writer on mechanics, Philo of Byzantium. In his work ''Pneumatica'', Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck.<ref>{{cite book|title = Story of Human Error|first = Joseph|last = Jastrow|url = https://books.google.com/books?id=tRUO45YfCHwC&pg=PA171|page = 171|date = 1936|publisher = Ayer Publishing|isbn = 978-0-8369-0568-7|access-date = August 23, 2020|archive-date = October 1, 2021|archive-url = https://web.archive.org/web/20211001032137/https://books.google.com/books?id=tRUO45YfCHwC&pg=PA171|url-status = live}}</ref> Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries later Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration.<ref name="ECE499">[[#Reference-idCook1968|Cook & Lauer 1968]], p. 499.</ref>

In the late 17th&nbsp;century, Robert Boyle proved that air is necessary for combustion. English chemist John Mayow (1641–1679) refined this work by showing that fire requires only a part of air that he called ''spiritus nitroaereus''.<ref name="EB1911">{{cite EB1911|wstitle=Mayow, John|volume=17|pages=938–39}}</ref> In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.<ref name="WoC">{{cite book|title=World of Chemistry|chapter=John Mayow|date=2005|publisher=Thomson Gale|chapter-url=http://www.bookrags.com/John_Mayow|access-date=December 16, 2007|isbn=978-0-669-32727-4|archive-date=April 17, 2020|archive-url=https://web.archive.org/web/20200417002720/http://www.bookrags.com/John_Mayow/|url-status=live}}</ref> From this, he surmised that nitroaereus is consumed in both respiration and combustion.

Mayow observed that antimony increased in weight when heated, and inferred that the nitroaereus must have combined with it.<ref name="EB1911" /> He also thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body.<ref name="EB1911" /> Accounts of these and other experiments and ideas were published in 1668 in his work ''Tractatus duo'' in the tract "De respiratione".<ref name="WoC" />

===Phlogiston theory===
{{Main|Phlogiston theory}}

Robert Hooke, Ole Borch, Mikhail Lomonosov, and Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element.<ref name="NBB299">[[#Reference-idEmsley2001|Emsley 2001]], p. 299</ref> This may have been in part due to the prevalence of the philosophy of combustion and corrosion called the ''phlogiston theory'', which was then the favored explanation of those processes.<ref>{{cite journal | last1 = Best | first1 = Nicholas W. | year = 2015 | title = Lavoisier's 'Reflections on Phlogiston' I: Against Phlogiston Theory | journal = Foundations of Chemistry | volume = 17 | issue = 2| pages = 137–51 | doi=10.1007/s10698-015-9220-5| s2cid = 170422925 }}</ref>

Established in 1667 by the German alchemist J. J. Becher, and modified by the chemist Georg Ernst Stahl by 1731,<ref name="morris">{{cite book| title = The last sorcerers: The path from alchemy to the periodic table
| url = https://archive.org/details/lastsorcererspat0000morr
| url-access = registration
| last = Morris| first = Richard| date = 2003|publisher = Joseph Henry Press|location = Washington, D.C.|isbn = 978-0-309-08905-0}}</ref> phlogiston theory stated that all combustible materials were made of two parts. One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx.<ref name="ECE499" />

Highly combustible materials that leave little residue, such as wood or coal, were thought to be made mostly of phlogiston; non-combustible substances that corrode, such as iron, contained very little. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea; instead, it was based on observations of what happens when something burns, that most common objects appear to become lighter and seem to lose something in the process.<ref name="ECE499" />

===Discovery===
[[File:PriestleyFuseli.jpg|thumb|upright|left|Joseph Priestley is usually given priority in the discovery.|alt=A drawing of an elderly man sitting by a table and facing parallel to the drawing. His left arm rests on a notebook, legs crossed.]]

Polish alchemist, philosopher, and physician Michael Sendivogius (Michał Sędziwój) in his work ''De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti'' ["Twelve Treatises on the Philosopher's Stone drawn from the source of nature and manual experience"] (1604) described a substance contained in air, referring to it as 'cibus vitae' (food of life,<ref name="Marples">{{cite web|last1=Marples|first1=Frater James A.|title=Michael Sendivogius, Rosicrucian, and Father of Studies of Oxygen|url=http://www.masonic.benemerito.net/msricf/papers/marples/marples-michael.sendivogius.pdf|publisher=Societas Rosicruciana in Civitatibus Foederatis, Nebraska College|access-date=2018-05-25|pages=3–4|archive-date=May 8, 2020|archive-url=https://web.archive.org/web/20200508172910/http://www.masonic.benemerito.net/msricf/papers/marples/marples-michael.sendivogius.pdf|url-status=live}}</ref>) and according to Polish historian Roman Bugaj, this substance is identical with oxygen.<ref name="Bugaj">{{cite journal |last1=Bugaj |first1=Roman |title=Michał Sędziwój – Traktat o Kamieniu Filozoficznym |journal=Biblioteka Problemów |date=1971 |volume=164 |pages=83–84 |url=https://books.google.com/books?id=d0gaAQAAMAAJ |language=pl |issn=0137-5032 |access-date=August 23, 2020 |archive-date=October 1, 2021 |archive-url=https://web.archive.org/web/20211001032100/https://books.google.com/books?id=d0gaAQAAMAAJ |url-status=live }}</ref> Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj's view, the isolation of oxygen and the proper association of the substance to that part of air which is required for life, provides sufficient evidence for the discovery of oxygen by Sendivogius.{{r|Bugaj}} This discovery of Sendivogius was however frequently denied by the generations of scientists and chemists which succeeded him.{{r|Marples}}

It is also commonly claimed that oxygen was first discovered by Swedish pharmacist Carl Wilhelm Scheele. He had produced oxygen gas by heating mercuric oxide (HgO) and various nitrates in 1771–72.<ref>{{cite web |url=http://www.rsc.org/periodic-table/element/8/oxygen |title=Oxygen |publisher=RSC.org |access-date=2016-12-12 |archive-date=January 28, 2017 |archive-url=https://web.archive.org/web/20170128145051/http://www.rsc.org/periodic-table/element/8/Oxygen |url-status=live }}</ref><ref name="ECE500" /><ref name="ECE499" /> Scheele called the gas "fire air" because it was then the only known agent to support combustion. He wrote an account of this discovery in a manuscript titled ''Treatise on Air and Fire'', which he sent to his publisher in 1775. That document was published in 1777.<ref name="NBB300">[[#Reference-idEmsley2001|Emsley 2001]], p. 300</ref>

In the meantime, on August 1, 1774, an experiment conducted by the British clergyman Joseph Priestley focused sunlight on mercuric oxide contained in a glass tube, which liberated a gas he named "dephlogisticated air".<ref name="ECE500">[[#Reference-idCook1968|Cook & Lauer 1968]], p. 500</ref> He noted that candles burned brighter in the gas and that a mouse was more active and lived longer while breathing it. After breathing the gas himself, Priestley wrote: "The feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards."<ref name="NBB299" /> Priestley published his findings in 1775 in a paper titled "An Account of Further Discoveries in Air", which was included in the second volume of his book titled ''Experiments and Observations on Different Kinds of Air''.<ref name="ECE499" /><ref>{{cite journal|title = An Account of Further Discoveries in Air|first = Joseph |last = Priestley |journal = Philosophical Transactions |date = 1775 |volume = 65 |pages = 384–94 |doi = 10.1098/rstl.1775.0039|doi-access = free }}</ref> Because he published his findings first, Priestley is usually given priority in the discovery.

The French chemist Antoine Laurent Lavoisier later claimed to have discovered the new substance independently. Priestley visited Lavoisier in October 1774 and told him about his experiment and how he liberated the new gas. Scheele had also dispatched a letter to Lavoisier on September 30, 1774, which described his discovery of the previously unknown substance, but Lavoisier never acknowledged receiving it (a copy of the letter was found in Scheele's belongings after his death).<ref name="NBB300" />

===Lavoisier's contribution===
[[File:Antoine lavoisier.jpg|thumb|upright|left|Antoine Lavoisier discredited the phlogiston theory.|alt=A drawing of a young man facing towards the viewer, but looking on the side. He wear a white curly wig, dark suit and white scarf.]]

Lavoisier conducted the first adequate quantitative experiments on oxidation and gave the first correct explanation of how combustion works.<ref name="ECE500" /> He used these and similar experiments, all started in 1774, to discredit the phlogiston theory and to prove that the substance discovered by Priestley and Scheele was a chemical element.

In one experiment, Lavoisier observed that there was no overall increase in weight when tin and air were heated in a closed container.<ref name="ECE500" /> He noted that air rushed in when he opened the container, which indicated that part of the trapped air had been consumed. He also noted that the tin had increased in weight and that increase was the same as the weight of the air that rushed back in. This and other experiments on combustion were documented in his book ''Sur la combustion en général'', which was published in 1777.<ref name="ECE500" /> In that work, he proved that air is a mixture of two gases; 'vital air', which is essential to combustion and respiration, and ''azote'' (Gk. ''{{lang|grc|ἄζωτον}}'' "lifeless"), which did not support either. ''Azote'' later became ''nitrogen'' in English, although it has kept the earlier name in French and several other European languages.<ref name="ECE500" />

====Etymology====
Lavoisier renamed 'vital air' to ''oxygène'' in 1777 from the Greek roots ''{{lang|grc|ὀξύς}} (oxys)'' (acid, literally 'sharp', from the taste of acids) and ''-γενής (-genēs)'' (producer, literally begetter), because he mistakenly believed that oxygen was a constituent of all acids.<ref name="mellor">{{cite book|last1=Parks|first1=G. D.|last2=Mellor|first2=J. W.|date=1939|title=Mellor's Modern Inorganic Chemistry|edition=6th |publisher=Longmans, Green and Co.|location=London}}</ref> Chemists (such as Sir Humphry Davy in 1812) eventually determined that Lavoisier was wrong in this regard, but by then the name was too well established.<ref>{{Greenwood&Earnshaw2nd|page=793}}</ref>

''Oxygen'' entered the English language despite opposition by English scientists and the fact that the Englishman Priestley had first isolated the gas and written about it. This is partly due to a poem praising the gas titled "Oxygen" in the popular book ''The Botanic Garden'' (1791) by Erasmus Darwin, grandfather of Charles Darwin.<ref name="NBB300" />
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===Later history===
[[File:Goddard and Rocket.jpg|thumb|upright|Robert H. Goddard and a liquid oxygen-gasoline rocket|alt=A metal frame structure stands on the snow near a tree. A middle-aged man wearing a coat, boots, leather gloves and a cap stands by the structure and holds it with his right hand.]]

John Dalton's original atomic hypothesis presumed that all elements were monatomic and that the atoms in compounds would normally have the simplest atomic ratios with respect to one another. For example, Dalton assumed that water's formula was HO, leading to the conclusion that the atomic mass of oxygen was 8 times that of hydrogen, instead of the modern value of about 16.<ref>{{cite book| title = The Interactive Textbook of PFP96 |chapter= Do We Take Atoms for Granted?|chapter-url=http://www.physics.upenn.edu/courses/gladney/mathphys/subsubsection1_1_3_2.html |url=http://www.physics.upenn.edu/courses/gladney/mathphys/Contents.html |first1=Dennis |last1=DeTurck |last2=Gladney|first2=Larry|last3=Pietrovito|first3=Anthony| publisher=University of Pennsylvania|date=1997|access-date=January 28, 2008|archive-url = https://web.archive.org/web/20080117230939/http://www.physics.upenn.edu/courses/gladney/mathphys/subsubsection1_1_3_2.html |archive-date = January 17, 2008|url-status=dead}}</ref> In 1805, Joseph Louis Gay-Lussac and Alexander von Humboldt showed that water is formed of two volumes of hydrogen and one volume of oxygen; and by 1811 Amedeo Avogadro had arrived at the correct interpretation of water's composition, based on what is now called Avogadro's law and the diatomic elemental molecules in those gases.<ref>{{cite book|title=A Treatise on Chemistry|first1=Henry Enfield |last1=Roscoe |last2=Schorlemmer|first2=Carl|page=38|date=1883|publisher=D. Appleton and Co.}}</ref><ref group=lower-alpha>These results were mostly ignored until 1860. Part of this rejection was due to the belief that atoms of one element would have no chemical affinity towards atoms of the same element, and part was due to apparent exceptions to Avogadro's law that were not explained until later in terms of dissociating molecules.</ref>

The first commercial method of producing oxygen was chemical, the so-called Brin process involving a reversible reaction of barium oxide. It was invented in 1852 and commercialized in 1884, but was displaced by newer methods in early 20th century.

By the late 19th century scientists realized that air could be liquefied and its components isolated by compressing and cooling it. Using a cascade method, Swiss chemist and physicist Raoul Pierre Pictet evaporated liquid sulfur dioxide in order to liquefy carbon dioxide, which in turn was evaporated to cool oxygen gas enough to liquefy it. He sent a telegram on December 22, 1877, to the French Academy of Sciences in Paris announcing his discovery of liquid oxygen.<ref name="BES707">{{cite book|title=Biographical Encyclopedia of Scientists|last=Daintith|first=John|date=1994|publisher=CRC Press|isbn=978-0-7503-0287-6|page=707}}</ref> Just two days later, French physicist Louis Paul Cailletet announced his own method of liquefying molecular oxygen.<ref name="BES707" /> Only a few drops of the liquid were produced in each case and no meaningful analysis could be conducted. Oxygen was liquefied in a stable state for the first time on March 29, 1883, by Polish scientists from Jagiellonian University, Zygmunt Wróblewski and Karol Olszewski.<ref>{{cite journal|title = Louis Paul Cailletet: The liquefaction of oxygen and the emergence of low-temperature research |first =Faidra |last = Papanelopoulou |journal =Notes and Records of the Royal Society of London |date = 2013|volume = 67 |issue=4|pages = 355–73|doi = 10.1098/rsnr.2013.0047 |pmc=3826198}}</ref>
[[File:A setup for preparation of Oxygen.jpg|alt=An experiment setup with test tubes to prepare oxygen|left|thumb|280x280px|An experiment setup for preparation of oxygen in academic laboratories]]
In 1891 Scottish chemist James Dewar was able to produce enough liquid oxygen for study.<ref name="NBB303">[[#Reference-idEmsley2001|Emsley 2001]], p. 303</ref> The first commercially viable process for producing liquid oxygen was independently developed in 1895 by German engineer Carl von Linde and British engineer William Hampson. Both men lowered the temperature of air until it liquefied and then distilled the component gases by boiling them off one at a time and capturing them separately.<ref name="HPAM">{{cite book|title=How Products are Made|chapter=Oxygen|publisher=The Gale Group, Inc.|date=2002|chapter-url=http://www.answers.com/topic/oxygen|access-date=December 16, 2007|archive-date=April 3, 2019|archive-url=https://web.archive.org/web/20190403220006/http://www.answers.com/topic/oxygen|url-status=live}}</ref> Later, in 1901, oxyacetylene welding was demonstrated for the first time by burning a mixture of acetylene and compressed {{chem|O|2}}. This method of welding and cutting metal later became common.<ref name="HPAM" />

In 1923, the American scientist Robert H. Goddard became the first person to develop a rocket engine that burned liquid fuel; the engine used gasoline for fuel and liquid oxygen as the oxidizer. Goddard successfully flew a small liquid-fueled rocket 56&nbsp;m at 97&nbsp;km/h on March 16, 1926, in Auburn, Massachusetts, US.<ref name="HPAM" /><ref>{{cite web|title=Goddard-1926 |url=http://grin.hq.nasa.gov/ABSTRACTS/GPN-2002-000132.html |publisher=NASA |access-date=November 18, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20071108225824/http://grin.hq.nasa.gov/ABSTRACTS/GPN-2002-000132.html |archive-date=November 8, 2007 }}</ref>

In academic laboratories, oxygen can be prepared by heating together potassium chlorate mixed with a small proportion of manganese dioxide.<ref>{{Cite book|url=https://archive.org/details/flescscho_1114918|title=A school chemistry|last=Flecker|first=Oriel Joyce|publisher=Oxford, Clarendon press|others=MIT Libraries|year=1924|page=[https://archive.org/details/flescscho_1114918/page/n41 30]}}</ref>

Oxygen levels in the atmosphere are trending slightly downward globally, possibly because of fossil-fuel burning.<ref>{{cite web|url=http://scrippso2.ucsd.edu/|title=Atmospheric Oxygen Research|author=Scripps Institute|access-date=October 8, 2011|archive-date=July 25, 2017|archive-url=https://web.archive.org/web/20170725074925/http://scrippso2.ucsd.edu/|url-status=live}}</ref>

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==Characteristics==
===Properties and molecular structure===
[[File:Oxygen molecule orbitals diagram-en.svg|thumb|left|upright=1.2|Orbital diagram, after Barrett (2002),<ref name="Barrett2002" /> showing the participating atomic orbitals from each oxygen atom, the molecular orbitals that result from their overlap, and the aufbau filling of the orbitals with the 12 electrons, 6 from each O atom, beginning from the lowest-energy orbitals, and resulting in covalent double-bond character from filled orbitals (and cancellation of the contributions of the pairs of σ and σ<sup>*</sup> and π and π<sup>*</sup> orbital pairs).]]
At standard temperature and pressure, oxygen is a colorless, odorless, and tasteless gas with the molecular formula {{chem|O|2}}, referred to as dioxygen.<ref>{{cite web |url=http://www.sciencekids.co.nz/sciencefacts/chemistry/oxygen.html |title=Oxygen Facts |publisher=Science Kids |date=February 6, 2015 |access-date=November 14, 2015 |archive-date=May 7, 2020 |archive-url=https://web.archive.org/web/20200507223541/https://www.sciencekids.co.nz/sciencefacts/chemistry/oxygen.html |url-status=live}}</ref>

As ''dioxygen'', two oxygen atoms are chemically bound to each other. The bond can be variously described based on level of theory, but is reasonably and simply described as a covalent double bond that results from the filling of molecular orbitals formed from the atomic orbitals of the individual oxygen atoms, the filling of which results in a bond order of two. More specifically, the double bond is the result of sequential, low-to-high energy, or Aufbau, filling of orbitals, and the resulting cancellation of contributions from the 2s electrons, after sequential filling of the low σ and σ<sup>*</sup> orbitals; σ overlap of the two atomic 2p orbitals that lie along the O–O molecular axis and π overlap of two pairs of atomic 2p orbitals perpendicular to the O–O molecular axis, and then cancellation of contributions from the remaining two 2p electrons after their partial filling of the π<sup>*</sup> orbitals.<ref name="Barrett2002">Jack Barrett, 2002, "Atomic Structure and Periodicity", (Basic concepts in chemistry, Vol.&nbsp;9 of Tutorial chemistry texts), Cambridge, UK: Royal Society of Chemistry, p.&nbsp;153, {{ISBN|0854046577}}. See [https://books.google.com/books?isbn=0854046577 Google Books]. {{Webarchive|url=https://web.archive.org/web/20200530044101/https://books.google.com/books?isbn=0854046577%2F |date=May 30, 2020 }} accessed January 31, 2015.</ref>

This combination of cancellations and σ and π overlaps results in dioxygen's double-bond character and reactivity, and a triplet electronic ground state. An electron configuration with two unpaired electrons, as is found in dioxygen orbitals (see the filled π* orbitals in the diagram) that are of equal energy—i.e., degenerate—is a configuration termed a spin triplet state. Hence, the ground state of the {{chem|O|2}} molecule is referred to as triplet oxygen.<ref name="BiochemOnline">{{cite web |work=Biochemistry Online |url=http://employees.csbsju.edu/hjakubowski/classes/ch331/oxphos/oldioxygenchem.html |title=Chapter 8: Oxidation-Phosphorylation, the Chemistry of Di-Oxygen |first=Henry |last=Jakubowski |access-date=January 28, 2008 |publisher=Saint John's University |archive-date=October 5, 2018 |archive-url=https://web.archive.org/web/20181005032115/http://employees.csbsju.edu/hjakubowski/classes/ch331/oxphos/oldioxygenchem.html |url-status=live}}</ref><ref group=lower-alpha>An orbital is a concept from quantum mechanics that models an electron as a wave-like particle that has a spatial distribution about an atom or molecule.</ref> The highest-energy, partially filled orbitals are antibonding, and so their filling weakens the bond order from three to two. Because of its unpaired electrons, triplet oxygen reacts only slowly with most organic molecules, which have paired electron spins; this prevents spontaneous combustion.<ref name="astm-tpt">{{cite conference|editor1-last=Werley|editor1-first=Barry L.|date=1991|work=Fire Hazards in Oxygen Systems|title=ASTM Technical Professional training|publisher=ASTM International Subcommittee G-4.05|location=Philadelphia}}</ref>

[[File:Liquid oxygen in a magnet 2.jpg|thumb|left|upright|Liquid oxygen, temporarily suspended in a magnet owing to its paramagnetism]]
In the triplet form, {{chem|O|2}} molecules are paramagnetic. That is, they impart magnetic character to oxygen when it is in the presence of a magnetic field, because of the spin magnetic moments of the unpaired electrons in the molecule, and the negative exchange energy between neighboring {{chem|O|2}} molecules.<ref name="NBB303" /> Liquid oxygen is so magnetic that, in laboratory demonstrations, a bridge of liquid oxygen may be supported against its own weight between the poles of a powerful magnet.<ref>{{cite web |url = http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/0809bondingpage/liquid_oxygen.htm |title = Demonstration of a bridge of liquid oxygen supported against its own weight between the poles of a powerful magnet |publisher = University of Wisconsin-Madison Chemistry Department Demonstration lab |access-date = December 15, 2007 |archive-url = https://web.archive.org/web/20071217064218/http://genchem.chem.wisc.edu/demonstrations/Gen_Chem_Pages/0809bondingpage/liquid_oxygen.htm |archive-date = December 17, 2007 |url-status=dead}}</ref>{{refn|Oxygen's paramagnetism can be used analytically in paramagnetic oxygen gas analysers that determine the purity of gaseous oxygen. ({{cite web |url=http://www.servomex.com/oxygen_gas_analyser.html |title=Company literature of Oxygen analyzers (triplet) |publisher=Servomex |access-date=December 15, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20080308213517/http://www.servomex.com/oxygen_gas_analyser.html |archive-date=March 8, 2008 }})|group=lower-alpha}}

Singlet oxygen is a name given to several higher-energy species of molecular {{chem|O|2}} in which all the electron spins are paired. It is much more reactive with common organic molecules than is normal (triplet) molecular oxygen. In nature, singlet oxygen is commonly formed from water during photosynthesis, using the energy of sunlight.<ref>{{cite journal |first=Anja |last=Krieger-Liszkay |journal=Journal of Experimental Botany |volume=56 |pages=337–346 |date=October 13, 2004 |title=Singlet oxygen production in photosynthesis |doi=10.1093/jxb/erh237 |pmid=15310815 |issue=411 |doi-access=free}}</ref> It is also produced in the troposphere by the photolysis of ozone by light of short wavelength<ref name="harrison">{{cite book |last=Harrison |first=Roy M. |author-link=Roy M. Harrison |date=1990 |title=Pollution: Causes, Effects & Control |edition=2nd |location=Cambridge |publisher=Royal Society of Chemistry |isbn=978-0-85186-283-5 |url-access=registration |url=https://archive.org/details/pollutioncausese0000unse}}</ref> and by the immune system as a source of active oxygen.<ref name="immune-ozone">{{cite journal |journal=Science |title=Evidence for Antibody-Catalyzed Ozone Formation in Bacterial Killing and Inflammation |date=December 13, 2002 |volume=298 |pages=2195–2219 |doi=10.1126/science.1077642 |pmid=12434011 |last1=Wentworth |first1=Paul |last2=McDunn |first2=J. E. |last3=Wentworth |first3=A. D. |last4=Takeuchi |first4=C. |last5=Nieva |first5=J. |last6=Jones |first6=T. |last7=Bautista |first7=C. |last8=Ruedi |first8=J. M. |last9=Gutierrez |first9=A. |last10=Janda |first10=K. D. |last11=Babior |first11=B. M. |last12=Eschenmoser |first12=A. |last13=Lerner |first13=R. A. |issue=5601 |bibcode=2002Sci...298.2195W |s2cid=36537588 |doi-access=free }}</ref> Carotenoids in photosynthetic organisms (and possibly animals) play a major role in absorbing energy from singlet oxygen and converting it to the unexcited ground state before it can cause harm to tissues.<ref>{{cite journal |title=Singlet oxygen quenching ability of naturally occurring carotenoids |journal=Lipids |first1=Osamu |last1=Hirayama |last2=Nakamura |first2=Kyoko |last3=Hamada |first3=Syoko |last4=Kobayasi |first4=Yoko |volume=29 |issue=2 |date=1994 |doi=10.1007/BF02537155 |pages=149–150 |pmid=8152349 |s2cid=3965039}}</ref>

===Allotropes===
{{Main|Allotropes of oxygen}}
[[File:Oxygen molecule.png|thumb|right|upright=0.9|Space-filling model representation of dioxygen (O<sub>2</sub>) molecule]]
The common allotrope of elemental oxygen on Earth is called dioxygen, {{chem|O|2}}, the major part of the Earth's atmospheric oxygen (see [[#Occurrence|Occurrence]]). O<sub>2</sub> has a bond length of 121&nbsp;pm and a bond energy of 498&nbsp;kJ/mol.<ref>{{cite web|last=Chieh|first=Chung|title=Bond Lengths and Energies|url=http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html|publisher=University of Waterloo|access-date=December 16, 2007|archive-url=https://web.archive.org/web/20071214215455/http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html|archive-date=December 14, 2007|url-status=dead}}</ref> O<sub>2</sub> is used by complex forms of life, such as animals, in cellular respiration. Other aspects of {{chem|O|2}} are covered in the remainder of this article.

Trioxygen ({{chem|O|3}}) is usually known as ozone and is a very reactive allotrope of oxygen that is damaging to lung tissue.<ref name="GuideElem48">{{cite book|title=Guide to the Elements|url=https://archive.org/details/guidetoelements00stwe|url-access=registration|edition=Revised |first=Albert|last=Stwertka|publisher=Oxford University Press|date=1998|isbn=978-0-19-508083-4|pages=[https://archive.org/details/guidetoelements00stwe/page/48 48–49]}}</ref> Ozone is produced in the upper atmosphere when {{chem|O|2}} combines with atomic oxygen made by the splitting of {{chem|O|2}} by ultraviolet (UV) radiation.<ref name="mellor" /> Since ozone absorbs strongly in the UV region of the spectrum, the ozone layer of the upper atmosphere functions as a protective radiation shield for the planet.<ref name="mellor" /> Near the Earth's surface, it is a pollutant formed as a by-product of automobile exhaust.<ref name="GuideElem48" /> At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft.<ref>{{cite web|access-date=August 8, 2009|url=http://www.spenvis.oma.be/spenvis/help/background/atmosphere/erosion.html|title=Atomic oxygen erosion|archive-url = https://web.archive.org/web/20070613121048/http://www.spenvis.oma.be/spenvis/help/background/atmosphere/erosion.html |archive-date = June 13, 2007|url-status=dead}}</ref>

The metastable molecule tetraoxygen ({{chem|O|4}}) was discovered in 2001,<ref name="o4">{{cite journal|last1=Cacace|first1=Fulvio|last2=de Petris|first2=Giulia|last3=Troiani|first3=Anna |date=2001|title=Experimental Detection of Tetraoxygen|journal=Angewandte Chemie International Edition|volume=40|issue=21|pages=4062–65|doi = 10.1002/1521-3773(20011105)40:21<4062::AID-ANIE4062>3.0.CO;2-X|pmid=12404493}}</ref><ref name="newform">{{cite news|first=Phillip|last=Ball|url=http://www.nature.com/news/2001/011122/pf/011122-3_pf.html|title=New form of oxygen found|work=Nature News|date=September 16, 2001|access-date=January 9, 2008|archive-date=October 21, 2013|archive-url=https://web.archive.org/web/20131021083801/http://www.nature.com/news/2001/011122/pf/011122-3_pf.html|url-status=live}}</ref> and was assumed to exist in one of the six phases of solid oxygen. It was proven in 2006 that this phase, created by pressurizing {{chem|O|2}} to 20&nbsp;GPa, is in fact a rhombohedral {{chem|O|8}} cluster.<ref>{{cite journal| title=Observation of an{{chem|O|8}} molecular lattice in the phase of solid oxygen|journal=Nature|volume=443|issue=7108|pages=201–04|doi=10.1038/nature05174|first1=Lars F. |last1=Lundegaard|pmid=16971946| display-authors=4| last2=Weck|first2=Gunnar|last3=McMahon|first3=Malcolm I.|last4=Desgreniers|first4=Serge|last5=Loubeyre|first5=Paul|date=2006|bibcode = 2006Natur.443..201L|s2cid=4384225}}</ref> This cluster has the potential to be a much more powerful oxidizer than either {{chem|O|2}} or {{chem|O|3}} and may therefore be used in rocket fuel.<ref name="o4" /><ref name="newform" /> A metallic phase was discovered in 1990 when solid oxygen is subjected to a pressure of above 96 GPa<ref>{{cite journal|last1=Desgreniers |first1=S. |last2=Vohra|first2=Y. K.|last3=Ruoff|first3=A. L.|title=Optical response of very high density solid oxygen to 132 GPa|journal=J. Phys. Chem.|volume=94|pages=1117–22|date=1990|doi=10.1021/j100366a020|issue=3}}</ref> and it was shown in 1998 that at very low temperatures, this phase becomes superconducting.<ref>{{cite journal|last1=Shimizu|first1=K.|display-authors=4|last2=Suhara|first2=K.|last3=Ikumo|first3=M.|last4=Eremets|first4=M. I.|last5= Amaya|first5=K.|title=Superconductivity in oxygen|journal=Nature|volume=393|pages=767–69|date=1998|doi=10.1038/31656|issue=6687|bibcode = 1998Natur.393..767S |s2cid=205001394|author4-link=Mikhail Eremets}}</ref>

===Physical properties===
[[File:Liquid oxygen in a beaker 4.jpg|thumb|Liquid oxygen boiling (O<sub>2</sub>)|alt=A transparent beaker containing a light blue fluid with gas bubbles.]]
{{see also|Liquid oxygen|solid oxygen}}
Oxygen dissolves more readily in water than nitrogen, and in freshwater more readily than in seawater. Water in equilibrium with air contains approximately 1 molecule of dissolved {{chem|O|2}} for every 2 molecules of {{chem|N|2}} (1:2), compared with an atmospheric ratio of approximately 1:4. The solubility of oxygen in water is temperature-dependent, and about twice as much ({{val|14.6|u=mg/L}}) dissolves at 0&nbsp;°C than at 20&nbsp;°C ({{val|7.6|u=mg/L}}).<ref name="NBB299" /><ref>{{cite web |url=http://www.engineeringtoolbox.com/air-solubility-water-d_639.html |title=Air solubility in water |access-date=December 21, 2007 |publisher=The Engineering Toolbox |archive-date=April 4, 2019 |archive-url=https://web.archive.org/web/20190404044017/https://www.engineeringtoolbox.com/air-solubility-water-d_639.html |url-status=live}}</ref> At 25&nbsp;°C and {{convert|1|atm|lk=on|sigfig=4}} of air, freshwater can dissolve about 6.04&nbsp;milliliters&nbsp;(mL) of oxygen per liter, and seawater contains about 4.95&nbsp;mL per liter.<ref>{{cite book |title = The Physiology of Fishes |first1=David Hudson |last1=Evans |last2=Claiborne |first2=James B. |page=88 |date=2005 |edition=3rd |publisher=CRC Press |isbn=978-0-8493-2022-4}}</ref> At 5&nbsp;°C the solubility increases to 9.0&nbsp;mL (50% more than at 25&nbsp;°C) per liter for freshwater and 7.2&nbsp;mL (45% more) per liter for sea water.
{| class="wikitable" style="float:left; margin-right:2em"
|+Oxygen gas dissolved in water at sea-level<br />(milliliters per liter)
!
!5&nbsp;°C
!25&nbsp;°C
|-
|Freshwater
|9.00
|6.04
|-
|Seawater
|7.20
|4.95
|}
Oxygen condenses at 90.20&nbsp;K (−182.95&nbsp;°C, −297.31&nbsp;°F) and freezes at 54.36&nbsp;K (−218.79&nbsp;°C, −361.82&nbsp;°F).<ref>{{cite book |first=David R. |last=Lide |title=CRC Handbook of Chemistry and Physics |edition=84th |publisher=CRC Press |location=Boca Raton, Florida |date=2003 |chapter=Section 4, Properties of the Elements and Inorganic Compounds; Melting, boiling, and critical temperatures of the elements |isbn=978-0-8493-0595-5 |url=https://archive.org/details/crchandbookofche0000unse_p1y5}}</ref> Both liquid and solid {{chem|O|2}} are clear substances with a light sky-blue color caused by absorption in the red (in contrast with the blue color of the sky, which is due to Rayleigh scattering of blue light). High-purity liquid {{chem|O|2}} is usually obtained by the fractional distillation of liquefied air.<ref>{{cite web |url = http://www.uigi.com/cryodist.html |title = Overview of Cryogenic Air Separation and Liquefier Systems |publisher = Universal Industrial Gases, Inc. |access-date = December 15, 2007 |archive-date = October 21, 2018 |archive-url = https://web.archive.org/web/20181021010346/http://www.uigi.com/cryodist.html |url-status = live}}</ref> Liquid oxygen may also be condensed from air using liquid nitrogen as a coolant.<ref name="LOX MSDS">{{cite web |url=https://www.mathesontrigas.com/pdfs/msds/00225011.pdf |title=Liquid Oxygen Material Safety Data Sheet |publisher=Matheson Tri Gas |access-date=December 15, 2007 |url-status=dead |archive-url=https://web.archive.org/web/20080227014309/https://www.mathesontrigas.com/pdfs/msds/00225011.pdf |archive-date=February 27, 2008 }}</ref>

Liquid oxygen is a highly reactive substance and must be segregated from combustible materials.<ref name="LOX MSDS" />

The spectroscopy of molecular oxygen is associated with the atmospheric processes of aurora and airglow.<ref name="Krupenie1972">{{cite journal |last1=Krupenie |first1=Paul H. |title=The Spectrum of Molecular Oxygen |journal=Journal of Physical and Chemical Reference Data |volume=1 |issue=2 |year=1972 |pages=423–534 |doi=10.1063/1.3253101 |bibcode=1972JPCRD...1..423K |s2cid=96242703 }}</ref> The absorption in the Herzberg continuum and Schumann–Runge bands in the ultraviolet produces atomic oxygen that is important in the chemistry of the middle atmosphere.<ref name="BrasseurSolomon2006">{{cite book |author1=Guy P. Brasseur |author2=Susan Solomon |title=Aeronomy of the Middle Atmosphere: Chemistry and Physics of the Stratosphere and Mesosphere |url=https://books.google.com/books?id=Z5OtlDjfXkkC&pg=PA220 |date=January 15, 2006 |publisher=Springer Science & Business Media |isbn=978-1-4020-3824-2 |pages=220– |access-date=July 2, 2015 |archive-date=February 2, 2017 |archive-url=https://web.archive.org/web/20170202143926/https://books.google.com/books?id=Z5OtlDjfXkkC&pg=PA220 |url-status=live}}</ref> Excited-state singlet molecular oxygen is responsible for red chemiluminescence in solution.<ref name="Kearns1971">{{cite journal |last1=Kearns |first1=David R. |title=Physical and chemical properties of singlet molecular oxygen |journal=Chemical Reviews |volume=71 |issue=4 |year=1971 |pages=395–427 |doi=10.1021/cr60272a004}}</ref>

Table of thermal and physical properties of oxygen (O<sub>2</sub>) at atmospheric pressure:<ref>{{Cite book |last=Holman |first=Jack P. |url=https://www.worldcat.org/oclc/46959719 |title=Heat transfer |publisher=McGraw-Hill Companies, Inc. |year=2002 |isbn=9780072406559 |edition=9th |location=New York, NY |pages=600–606 |language=English |oclc=46959719}}</ref><ref>{{Cite book |last=Incropera 1 Dewitt 2 Bergman 3 Lavigne 4 |first=Frank P. 1 David P. 2 Theodore L. 3 Adrienne S. 4 |url=https://www.worldcat.org/oclc/62532755 |title=Fundamentals of heat and mass transfer. |publisher=John Wiley and Sons, Inc. |year=2007 |isbn=9780471457282 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English |oclc=62532755}}</ref>
{|class="wikitable mw-collapsible mw-collapsed"
|Temperature (K)
|Density (kg/m^3)
|Specific heat (kJ/kg °C)
|Dynamic viscosity (kg/m s)
|Kinematic viscosity (m^2/s)
|Thermal conductivity (W/m °C)
|Thermal diffusivity (m^2/s)
|Prandtl Number
|-
|100
|3.945
|0.962
|7.64E-06
|1.94E-06
|0.00925
|2.44E-06
|0.796
|-
|150
|2.585
|0.921
|1.15E-05
|4.44E-06
|0.0138
|5.80E-06
|0.766
|-
|200
|1.93
|0.915
|1.48E-05
|7.64E-06
|0.0183
|1.04E-05
|0.737
|-
|250
|1.542
|0.915
|1.79E-05
|1.16E-05
|0.0226
|1.60E-05
|0.723
|-
|300
|1.284
|0.92
|2.07E-05
|1.61E-05
|0.0268
|2.27E-05
|0.711
|-
|350
|1.1
|0.929
|2.34E-05
|2.12E-05
|0.0296
|2.90E-05
|0.733
|-
|400
|0.962
|1.0408
|2.58E-05
|2.68E-05
|0.033
|3.64E-05
|0.737
|-
|450
|0.8554
|0.956
|2.81E-05
|3.29E-05
|0.0363
|4.44E-05
|0.741
|-
|500
|0.7698
|0.972
|3.03E-05
|3.94E-05
|0.0412
|5.51E-05
|0.716
|-
|550
|0.6998
|0.988
|3.24E-05
|4.63E-05
|0.0441
|6.38E-05
|0.726
|-
|600
|0.6414
|1.003
|3.44E-05
|5.36E-05
|0.0473
|7.35E-05
|0.729
|-
|700
|0.5498
|1.031
|3.81E-05
|6.93E-05
|0.0528
|9.31E-05
|0.744
|-
|800
|0.481
|1.054
|4.15E-05
|8.63E-05
|0.0589
|1.16E-04
|0.743
|-
|900
|0.4275
|1.074
|4.47E-05
|1.05E-04
|0.0649
|1.41E-04
|0.74
|-
|1000
|0.3848
|1.09
|4.77E-05
|1.24E-04
|0.071
|1.69E-04
|0.733
|-
|1100
|0.3498
|1.103
|5.06E-05
|1.45E-04
|0.0758
|1.96E-04
|0.736
|-
|1200
|0.3206
|1.0408
|5.33E-05
|1.661E-04
|0.0819
|2.29E-04
|0.725
|-
|1300
|0.296
|1.125
|5.88E-05
|1.99E-04
|0.0871
|2.62E-04
|0.721
|}

===Isotopes and stellar origin===
<!--COPYEDITS AND CORRECTIONS ONLY: DIRECT EXPANSION OF THIS SUBTOPIC TO Isotopes of oxygen -->
{{Main|Isotopes of oxygen}}
[[File:Evolved star fusion shells.svg|thumb|Late in a massive star's life, <sup>16</sup>O concentrates in the O-shell, <sup>17</sup>O in the H-shell and <sup>18</sup>O in the He-shell.|alt=A concentric-sphere diagram, showing, from the core to the outer shell, iron, silicon, oxygen, neon, carbon, helium and hydrogen layers.]]
Naturally occurring oxygen is composed of three stable isotopes, <sup>16</sup>O, <sup>17</sup>O, and <sup>18</sup>O, with <sup>16</sup>O being the most abundant (99.762% natural abundance).<ref name="EnvChem-Iso">{{cite web|url=http://environmentalchemistry.com/yogi/periodic/O-pg2.html|title=Oxygen Nuclides / Isotopes|publisher=EnvironmentalChemistry.com|access-date=December 17, 2007|archive-date=July 12, 2012|archive-url=https://archive.today/20120712195516/http://environmentalchemistry.com/yogi/periodic/O-pg2.html|url-status=live}}</ref>

Most <sup>16</sup>O is synthesized at the end of the helium fusion process in massive stars but some is made in the neon burning process.<ref name="Meyer2005">{{cite conference|first=B. S.|last=Meyer|title=Nucleosynthesis and Galactic Chemical Evolution of the Isotopes of Oxygen|conference=Workgroup on Oxygen in the Earliest Solar System|date=September 19–21, 2005|location=Gatlinburg, Tennessee|url=http://www.lpi.usra.edu/meetings/ess2005/pdf/9022.pdf|access-date=January 22, 2007|work=Proceedings of the NASA Cosmochemistry Program and the Lunar and Planetary Institute|conference-url=http://www.lpi.usra.edu/meetings/ess2005/|id=9022|archive-date=December 29, 2010|archive-url=https://web.archive.org/web/20101229194925/http://www.lpi.usra.edu/meetings/ess2005/pdf/9022.pdf|url-status=live}}</ref> <sup>17</sup>O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars.<ref name="Meyer2005" /> Most <sup>18</sup>O is produced when <sup>14</sup>N (made abundant from CNO burning) captures a <sup>4</sup>He nucleus, making <sup>18</sup>O common in the helium-rich zones of evolved, massive stars.<ref name="Meyer2005" />

Fifteen radioisotopes have been characterized, ranging from <sup>11</sup>O to <sup>28</sup>O.{{NUBASE2020|ref}}<ref name=O-28-SA>{{cite news |url=https://www.sciencealert.com/scientists-have-observed-a-never-before-seen-form-of-oxygen |first=Michelle |last=Starr |date=30 August 2023 |title=Scientists Have Observed A Never-Before-Seen Form of Oxygen |work=ScienceAlert |access-date=30 August 2023}}</ref> The most stable are <sup>15</sup>O with a half-life of 122.24&nbsp;seconds and <sup>14</sup>O with a half-life of 70.606&nbsp;seconds.<ref name="EnvChem-Iso" /> All of the remaining radioactive isotopes have half-lives that are less than 27&nbsp;seconds and the majority of these have half-lives that are less than 83&nbsp;milliseconds.<ref name="EnvChem-Iso" /> The most common decay mode of the isotopes lighter than <sup>16</sup>O is β<sup>+</sup> decay<ref name="NUDAT-13O">{{cite web|url=http://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=13O&unc=nds|title=NUDAT 13O|access-date=July 6, 2009|archive-date=June 9, 2022|archive-url=https://web.archive.org/web/20220609000104/http://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=13O|url-status=live}}</ref><ref name="NUDAT-14O">{{cite web|url=http://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=14O&unc=nds|title=NUDAT 14O|access-date=July 6, 2009|archive-date=June 7, 2022|archive-url=https://web.archive.org/web/20220607045357/http://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=14O|url-status=live}}</ref><ref name="NUDAT-15O">{{cite web|url=http://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=15O&unc=nds|title=NUDAT 15O|access-date=July 6, 2009|archive-date=June 7, 2022|archive-url=https://web.archive.org/web/20220607045434/http://www.nndc.bnl.gov/nudat2/decaysearchdirect.jsp?nuc=15O|url-status=live}}</ref> to yield nitrogen, and the most common mode for the isotopes heavier than <sup>18</sup>O is beta decay to yield fluorine.<ref name="EnvChem-Iso" />

===Occurrence===
{{see also|Silicate minerals|Category:Oxide minerals|Stellar population|Cosmochemistry|Astrochemistry}}
{| class="wikitable sortable" style="float:left; margin-right: 20px"
|+Ten most common elements in the Milky Way Galaxy estimated spectroscopically (not to scale)<ref name="croswell">{{cite book | last = Croswell | first = Ken | title = Alchemy of the Heavens | publisher = Anchor | year = 1996 | url = http://kencroswell.com/alchemy.html | isbn = 978-0-385-47214-2 | access-date = December 2, 2011 | archive-date = May 13, 2011 | archive-url = https://web.archive.org/web/20110513233910/http://www.kencroswell.com/alchemy.html | url-status = live }}</ref>
|-
!Z !! Element !! colspan="2"|Mass fraction in parts per million
|-
| 1 || Hydrogen || align="right"|{{bartable| 739,000||0.001}}
|-
| 2 || Helium || align="right"|{{bartable| 240,000||0.001}}
|-
| 8 || Oxygen || align="right"|{{bartable| 10,400||0.005||background:red;}}
|-
| 6 || Carbon || align="right"|{{bartable| 4,600||0.005}}
|-
| 10 || Neon || align="right"|{{bartable| 1,340||0.005}}
|-
| 26 || Iron || align="right"|{{bartable| 1,090||0.005}}
|-
| 7 || Nitrogen || align="right"|{{bartable| 960||0.005}}
|-
| 14 || Silicon || align="right"|{{bartable| 650||0.005}}
|-
| 12 || Magnesium || align="right"|{{bartable| 580||0.005}}
|-
| 16 || Sulfur || align="right"|{{bartable| 440||0.005}}
|}

Oxygen is the most abundant chemical element by mass in the Earth's biosphere, air, sea and land. Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium.<ref name="NBB297">[[#Reference-idEmsley2001|Emsley 2001]], p. 297</ref> About 0.9% of the Sun's mass is oxygen.<ref name="ECE500" /> Oxygen constitutes 49.2% of the Earth's crust by mass<ref name="lanl">{{cite web |url=http://periodic.lanl.gov/elements/8.html|publisher=Los Alamos National Laboratory|title=Oxygen|access-date=December 16, 2007|archive-url=https://web.archive.org/web/20071026034224/http://periodic.lanl.gov/elements/8.html|archive-date=October 26, 2007}}</ref> as part of oxide compounds such as silicon dioxide and is the most abundant element by mass in the Earth's crust. It is also the major component of the world's oceans (88.8% by mass).<ref name="ECE500" /> Oxygen gas is the second most common component of the Earth's atmosphere, taking up 20.8% of its volume and 23.1% of its mass (some 10<sup>15</sup> tonnes).<ref name="ECE500" /><ref name="NBB298">[[#Reference-idEmsley2001|Emsley 2001]], p. 298</ref><ref group="lower-alpha">Figures given are for values up to {{convert|80|km|mi|abbr=on}} above the surface</ref> Earth is unusual among the planets of the Solar System in having such a high concentration of oxygen gas in its atmosphere: Mars (with 0.1% {{chem|O|2}} by volume) and Venus have much less. The {{chem|O|2}} surrounding those planets is produced solely by the action of ultraviolet radiation on oxygen-containing molecules such as carbon dioxide.

[[File:WOA09 sea-surf O2 AYool.png|thumb|right|Cold water holds more dissolved {{chem|O|2}}.|alt=World map showing that the sea-surface oxygen is depleted around the equator and increases towards the poles.]]

The unusually high concentration of oxygen gas on Earth is the result of the oxygen cycle. This biogeochemical cycle describes the movement of oxygen within and between its three main reservoirs on Earth: the atmosphere, the biosphere, and the lithosphere. The main driving factor of the oxygen cycle is photosynthesis, which is responsible for modern Earth's atmosphere. Photosynthesis releases oxygen into the atmosphere, while respiration, decay, and combustion remove it from the atmosphere. In the present equilibrium, production and consumption occur at the same rate.<ref>{{Greenwood&Earnshaw2nd|page=602}}</ref>

Free oxygen also occurs in solution in the world's water bodies. The increased solubility of {{chem|O|2}} at lower temperatures (see [[#Physical properties|Physical properties]]) has important implications for ocean life, as polar oceans support a much higher density of life due to their higher oxygen content.<ref>From The Chemistry and Fertility of Sea Waters by H.W. Harvey, 1955, citing C.J.J. Fox, "On the coefficients of absorption of atmospheric gases in sea water", Publ. Circ. Cons. Explor. Mer, no. 41, 1907. Harvey notes that according to later articles in ''Nature'', the values appear to be about 3% too high.</ref> Water polluted with plant nutrients such as nitrates or phosphates may stimulate growth of algae by a process called eutrophication and the decay of these organisms and other biomaterials may reduce the {{chem|O|2}} content in eutrophic water bodies. Scientists assess this aspect of water quality by measuring the water's biochemical oxygen demand, or the amount of {{chem|O|2}} needed to restore it to a normal concentration.<ref name="NBB301">[[#Reference-idEmsley2001|Emsley 2001]], p. 301</ref>

===Analysis===
[[File:Phanerozoic Climate Change.png|thumb|left|upright=1.15|500 million years of climate change vs. <sup>18</sup>O|alt=Time evolution of oxygen-18 concentration on the scale of 500 million years showing many local peaks.]]

Paleoclimatologists measure the ratio of oxygen-18 and oxygen-16 in the shells and skeletons of marine organisms to determine the climate millions of years ago (see oxygen isotope ratio cycle). Seawater molecules that contain the lighter isotope, oxygen-16, evaporate at a slightly faster rate than water molecules containing the 12% heavier oxygen-18, and this disparity increases at lower temperatures.<ref name="NBB304">[[#Reference-idEmsley2001|Emsley 2001]], p. 304</ref> During periods of lower global temperatures, snow and rain from that evaporated water tends to be higher in oxygen-16, and the seawater left behind tends to be higher in oxygen-18. Marine organisms then incorporate more oxygen-18 into their skeletons and shells than they would in a warmer climate.<ref name="NBB304" /> Paleoclimatologists also directly measure this ratio in the water molecules of ice core samples as old as hundreds of thousands of years.

Planetary geologists have measured the relative quantities of oxygen isotopes in samples from the Earth, the Moon, Mars, and meteorites, but were long unable to obtain reference values for the isotope ratios in the Sun, believed to be the same as those of the primordial solar nebula. Analysis of a silicon wafer exposed to the solar wind in space and returned by the crashed Genesis spacecraft has shown that the Sun has a higher proportion of oxygen-16 than does the Earth. The measurement implies that an unknown process depleted oxygen-16 from the Sun's disk of protoplanetary material prior to the coalescence of dust grains that formed the Earth.<ref>{{cite journal|last = Hand|first = Eric|title = The Solar System's first breath|journal = Nature|volume = 452|page = 259|date = March 13, 2008|doi = 10.1038/452259a|pmid = 18354437|issue = 7185|bibcode = 2008Natur.452..259H |s2cid = 789382|doi-access = free}}</ref>

Oxygen presents two spectrophotometric absorption bands peaking at the wavelengths 687 and 760&nbsp;nm. Some remote sensing scientists have proposed using the measurement of the radiance coming from vegetation canopies in those bands to characterize plant health status from a satellite platform.<ref>{{cite conference|title=Progress on the development of an integrated canopy fluorescence model|last1=Miller|first1=J. R.|display-authors=4|author2=Berger, M.|author3=Alonso, L.|author4=Cerovic, Z.|author5=Goulas, Y.|author6=Jacquemoud, S.|author7=Louis, J.|author8=Mohammed, G.|author9=Moya, I.|author10=Pedros, R.|author11=Moreno, J.F.|author12=Verhoef, W.|author13=Zarco-Tejada, P.J.|work=Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International|year=2003 |volume=1 |pages=601–603 |doi=10.1109/IGARSS.2003.1293855|isbn=0-7803-7929-2 |citeseerx=10.1.1.473.9500}}</ref> This approach exploits the fact that in those bands it is possible to discriminate the vegetation's reflectance from its fluorescence, which is much weaker. The measurement is technically difficult owing to the low signal-to-noise ratio and the physical structure of vegetation; but it has been proposed as a possible method of monitoring the carbon cycle from satellites on a global scale.
{{Clear}}

==Biological production and role of O<sub>2</sub>==
{{Main|Dioxygen in biological reactions}}
<!-- CopyEdits Only&nbsp;— DIRECT ALL FUTURE EXPANSION to dioxygen in biological reactions -->

===Photosynthesis and respiration===
<!-- CopyEdits Only&nbsp;— DIRECT ALL FUTURE EXPANSION to dioxygen in biological reactions -->
[[File:Simple photosynthesis overview.svg|thumb|Photosynthesis splits water to liberate {{chem|O|2}} and fixes {{chem|CO|2}} into sugar in what is called a Calvin cycle.|alt=A diagram of photosynthesis processes, including income of water and carbon dioxide, illumination and release of oxygen. Reactions produce ATP and NADPH in a Calvin cycle with a sugar as a by product.]]
In nature, free oxygen is produced by the light-driven splitting of water during oxygenic photosynthesis. According to some estimates, green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth, and the rest is produced by terrestrial plants.<ref>{{cite book|chapter-url=https://books.google.com/books?id=g6RfkqCUQyQC&pg=PA147|title=Plants: the potentials for extracting protein, medicines, and other useful chemicals (workshop proceedings)|date=September 1983|chapter=Marine Plants: A Unique and Unexplored Resource|last=Fenical|first=William|page=147|isbn=978-1-4289-2397-3|publisher=DianePublishing|access-date=August 23, 2020|archive-date=March 25, 2015|archive-url=https://web.archive.org/web/20150325221600/http://books.google.com/books?id=g6RfkqCUQyQC&pg=PA147|url-status=live}}</ref> Other estimates of the oceanic contribution to atmospheric oxygen are higher, while some estimates are lower, suggesting oceans produce ~45% of Earth's atmospheric oxygen each year.<ref>{{cite book|last=Walker|first=J. C. G.|date=1980|title=The oxygen cycle in the natural environment and the biogeochemical cycles|publisher=Springer-Verlag|location=Berlin}}</ref>

A simplified overall formula for photosynthesis is<ref>{{cite book|last1=Brown|first1=Theodore L. |last2=LeMay|first2=Burslen|title=Chemistry: The Central Science|url=https://archive.org/details/studentlectureno00theo|url-access=registration|isbn=978-0-13-048450-5|page=958|date=2003|publisher=Prentice Hall/Pearson Education}}</ref>

: 6 {{CO2}} + 6 {{chem|H|2|O}} + photons → {{chem|C|6|H|12|O|6}} + 6 {{chem|O|2}}

or simply

: carbon dioxide + water + sunlight → glucose + dioxygen

Photolytic oxygen evolution occurs in the thylakoid membranes of photosynthetic organisms and requires the energy of four photons.<ref group=lower-alpha>Thylakoid membranes are part of chloroplasts in algae and plants while they simply are one of many membrane structures in cyanobacteria. In fact, chloroplasts are thought to have evolved from cyanobacteria that were once symbiotic partners with the progenitors of plants and algae.</ref> Many steps are involved, but the result is the formation of a proton gradient across the thylakoid membrane, which is used to synthesize adenosine triphosphate (ATP) via photophosphorylation.<ref name="Raven">[[#Reference-idRaven2005|Raven 2005]], 115–27</ref> The {{chem|O|2}} remaining (after production of the water molecule) is released into the atmosphere.<ref group=lower-alpha>Water oxidation is catalyzed by a manganese-containing enzyme complex known as the oxygen evolving complex (OEC) or water-splitting complex found associated with the lumenal side of thylakoid membranes. Manganese is an important cofactor, and calcium and chloride are also required for the reaction to occur. (Raven 2005)</ref>

Oxygen is used in mitochondria in the generation of ATP during oxidative phosphorylation. The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified as

: {{chem|C|6|H|12|O|6}} + 6 {{chem|O|2}} → 6 {{CO2}} + 6 {{chem|H|2|O}} + 2880 kJ/mol

In vertebrates, {{chem|O|2}} diffuses through membranes in the lungs and into red blood cells. Hemoglobin binds {{chem|O|2}}, changing color from bluish red to bright red<ref name="GuideElem48" /> ({{chem|CO|2}} is released from another part of hemoglobin through the Bohr effect). Other animals use hemocyanin (molluscs and some arthropods) or hemerythrin (spiders and lobsters).<ref name="NBB298" /> A liter of blood can dissolve 200&nbsp;cm<sup>3</sup> of {{chem|O|2}}.<ref name="NBB298" />

Until the discovery of anaerobic metazoa,<ref name="pmid20370908">{{cite journal |display-authors=4 |author=Danovaro R |author2=Dell'anno A |author3=Pusceddu A|author4=Gambi C |author5=Heiner I|author6=Kristensen RM |title=The first metazoa living in permanently anoxic conditions |journal=BMC Biology |volume=8 |issue=1 |pages=30 |date=April 2010 |pmid=20370908 |pmc=2907586 |doi=10.1186/1741-7007-8-30 |doi-access=free}}</ref> oxygen was thought to be a requirement for all complex life.<ref>{{cite book |last1=Ward |first1=Peter D. |last2=Brownlee |first2=Donald |title=Rare Earth: Why Complex Life is Uncommon in the Universe |publisher=Copernicus Books (Springer Verlag) |date=2000 |isbn=978-0-387-98701-9 |page=217}}</ref>

Reactive oxygen species, such as superoxide ion ({{chem|O|2|-}}) and hydrogen peroxide ({{chem|H|2|O|2}}), are reactive by-products of oxygen use in organisms.<ref name="NBB298" /> Parts of the immune system of higher organisms create peroxide, superoxide, and singlet oxygen to destroy invading microbes. Reactive oxygen species also play an important role in the hypersensitive response of plants against pathogen attack.<ref name="Raven" /> Oxygen is damaging to obligately anaerobic organisms, which were the dominant form of early life on Earth until {{chem|O|2}} began to accumulate in the atmosphere about 2.5 billion years ago during the Great Oxygenation Event, about a billion years after the first appearance of these organisms.<ref>{{cite press release |title=NASA Research Indicates Oxygen on Earth 2.5 Billion Years ago |url=http://www.nasa.gov/home/hqnews/2007/sep/HQ_07215_Timeline_of_Oxygen_on_Earth.html |publisher=NASA |date=September 27, 2007 |access-date=March 13, 2008 |archive-date=March 13, 2008 |archive-url=https://web.archive.org/web/20080313063940/http://www.nasa.gov/home/hqnews/2007/sep/HQ_07215_Timeline_of_Oxygen_on_Earth.html |url-status=live }}</ref><ref name="NYT-20131003">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=Earth's Oxygen: A Mystery Easy to Take for Granted |url=https://www.nytimes.com/2013/10/03/science/earths-oxygen-a-mystery-easy-to-take-for-granted.html |date=October 3, 2013 |work=The New York Times |access-date=October 3, 2013 |archive-date=May 16, 2020 |archive-url=https://web.archive.org/web/20200516083101/https://www.nytimes.com/2013/10/03/science/earths-oxygen-a-mystery-easy-to-take-for-granted.html |url-status=live }}</ref>

An adult human at rest inhales<!--simply inhales (most is exhaled again) or takes up and respires?--> 1.8 to 2.4&nbsp;grams of oxygen per minute.<ref>{{Cite web|url=https://patents.google.com/patent/US6224560B1/en|title=Flow restrictor for measuring respiratory parameters|access-date=August 4, 2019|archive-date=May 8, 2020|archive-url=https://web.archive.org/web/20200508103811/https://patents.google.com/patent/US6224560B1/en|url-status=live}}</ref> This amounts to more than 6 billion tonnes of oxygen inhaled by humanity per year.<ref group=lower-alpha>(1.8 grams/min/person)×(60 min/h)×(24 h/day)×(365 days/year)×(6.6 billion people)/1,000,000 g/t=6.24 billion tonnes</ref>

===Living organisms===
{{anchor|partial pressure}}
{|class="wikitable" style="float:right; margin-left:25px"
|+Partial pressures of oxygen in the human body (PO<sub>2</sub>)
|-
! Unit !! Alveolar pulmonary<br /> gas pressures !! Arterial blood oxygen !! Venous blood gas
|-
| kPa || 14.2 || 11{{efn|name=mmHg|Derived from mmHg values using 0.133322 kPa/mmHg}}-13{{efn|name=mmHg}} || 4.0{{efn|name=mmHg}}-5.3{{efn|name=mmHg}}
|-
| mmHg || 107 || 75<ref name="southwest">
[http://pathcuric1.swmed.edu/PathDemo/nrrt.htm Normal Reference Range Table] {{Webarchive|url=https://web.archive.org/web/20111225185659/http://pathcuric1.swmed.edu/PathDemo/nrrt.htm |date=December 25, 2011 }} from The University of Texas Southwestern Medical Center at Dallas. Used in Interactive Case Study Companion to Pathologic basis of disease.
</ref>-100<ref name="southwest" /> || 30<ref name="brookside" />-40<ref name="brookside">[http://www.brooksidepress.org/Products/OperationalMedicine/DATA/operationalmed/Lab/ABG_ArterialBloodGas.htm The Medical Education Division of the Brookside Associates--> ABG (Arterial Blood Gas)] {{Webarchive|url=https://web.archive.org/web/20170812201558/http://www.brooksidepress.org/Products/OperationalMedicine/DATA/operationalmed/Lab/ABG_ArterialBloodGas.htm |date=August 12, 2017 }} Retrieved on December 6, 2009</ref>
|-
|}
The free oxygen partial pressure in the body of a living vertebrate organism is highest in the respiratory system, and decreases along any arterial system, peripheral tissues, and venous system, respectively. Partial pressure is the pressure that oxygen would have if it alone occupied the volume.<ref>{{cite book|author=Charles Henrickson|title=Chemistry|publisher=Cliffs Notes|date=2005|isbn=978-0-7645-7419-1|url=https://archive.org/details/chemistry00henr}}</ref>

===Build-up in the atmosphere===
{{Main|Geological history of oxygen}}
<!-- CopyEdits Only&nbsp;— DIRECT ALL FUTURE EXPANSION to Geological history of oxygen or dioxygen in biological reactions -->

[[File:Oxygenation-atm.svg|thumb|left|upright=1.35|{{chem|O|2}} build-up in Earth's atmosphere: 1) no {{chem|O|2}} produced; 2) {{chem|O|2}} produced, but absorbed in oceans & seabed rock; 3) {{chem|O|2}} starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4–5) {{chem|O|2}} sinks filled and the gas accumulates|alt=A graph showing time evolution of oxygen pressure on Earth; the pressure increases from zero to 0.2 atmospheres.]]

Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteria evolved, probably about 3.5 billion years ago. Free oxygen first appeared in significant quantities during the Paleoproterozoic era (between 3.0 and 2.3 billion years ago).<ref name="Crowe2013">{{Cite journal | last1 = Crowe | first1 = S. A. | last2 = Døssing | first2 = L. N. | last3 = Beukes | first3 = N. J. | last4 = Bau | first4 = M. | last5 = Kruger | first5 = S. J. | last6 = Frei | first6 = R. | last7 = Canfield | first7 = D. E. | title = Atmospheric oxygenation three billion years ago | journal = Nature | volume = 501 | issue = 7468 | pages = 535–38 | year = 2013 | pmid = 24067713 | doi = 10.1038/nature12426 | bibcode = 2013Natur.501..535C | s2cid = 4464710 }}</ref> Even if there was much dissolved iron in the oceans when oxygenic photosynthesis was getting more common, it appears the banded iron formations were created by anoxyenic or micro-aerophilic iron-oxidizing bacteria which dominated the deeper areas of the photic zone, while oxygen-producing cyanobacteria covered the shallows.<ref>[https://www.sciencedaily.com/releases/2013/04/130423110750.htm Iron in primeval seas rusted by bacteria] {{Webarchive|url=https://web.archive.org/web/20200311023339/https://www.sciencedaily.com/releases/2013/04/130423110750.htm |date=March 11, 2020 }}, ScienceDaily, April 23, 2013</ref> Free oxygen began to outgas from the oceans 3–2.7&nbsp;billion years ago, reaching 10% of its present level around 1.7&nbsp;billion years ago.<ref name="Crowe2013" /><ref name="Campbell">{{cite book|last1 = Campbell|first1 = Neil A.|last2=Reece|first2=Jane B.|title = Biology|edition = 7th|publisher = Pearson&nbsp;– Benjamin Cummings |date=2005|location = San Francisco|pages = 522–23|isbn = 978-0-8053-7171-0}}</ref>

The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the extant anaerobic organisms to extinction during the Great Oxygenation Event (''oxygen catastrophe'') about 2.4 billion years ago. Cellular respiration using {{chem|O|2}} enables aerobic organisms to produce much more ATP than anaerobic organisms.<ref name="Freeman">{{cite book|last = Freeman|first = Scott|title = Biological Science, 2nd|publisher = Pearson&nbsp;– Prentice Hall|date = 2005|location = Upper Saddle River, NJ|pages = [https://archive.org/details/biologicalscienc00scot/page/214 214, 586]|isbn = 978-0-13-140941-5|url = https://archive.org/details/biologicalscienc00scot/page/214}}</ref> Cellular respiration of {{chem|O|2}} occurs in all eukaryotes, including all complex multicellular organisms such as plants and animals.

Since the beginning of the Cambrian period 540 million years ago, atmospheric {{chem|O|2}} levels have fluctuated between 15% and 30% by volume.<ref name="geologic">{{cite journal |title=Atmospheric oxygen over Phanerozoic time |first=Robert A. |last=Berner |issue=20 |pages=10955–57 |date=1999|journal=Proceedings of the National Academy of Sciences of the USA |pmid=10500106 |doi=10.1073/pnas.96.20.10955 |volume=96 |pmc=34224 |bibcode=1999PNAS...9610955B|doi-access=free }}</ref> Towards the end of the Carboniferous period (about 300&nbsp;million years ago) atmospheric {{chem|O|2}} levels reached a maximum of 35% by volume,<ref name="geologic" /> which may have contributed to the large size of insects and amphibians at this time.<ref name="Butterfield2009">{{Cite journal | last1 = Butterfield | first1 = N. J. | title = Oxygen, animals and oceanic ventilation: An alternative view | doi = 10.1111/j.1472-4669.2009.00188.x | journal = Geobiology | volume = 7 | issue = 1 | pages = 1–7 | year = 2009 | pmid = 19200141 | bibcode = 2009Gbio....7....1B | s2cid = 31074331 }}</ref>

Variations in atmospheric oxygen concentration have shaped past climates. When oxygen declined, atmospheric density dropped, which in turn increased surface evaporation, causing precipitation increases and warmer temperatures.<ref>{{cite journal|url=http://ns.umich.edu/new/releases/22942-variations-in-atmospheric-oxygen-levels-shaped-earth-s-climate-through-the-ages|doi=10.1126/science.1260670|pmid=26068848|journal=Science|title=Long-term climate forcing by atmospheric oxygen concentrations|author1=Poulsen, Christopher J.|author2=Tabor, Clay|author3=White, Joseph D.|volume=348|issue=6240|pages=1238–41|bibcode=2015Sci...348.1238P|year=2015|s2cid=206562386|access-date=June 12, 2015|archive-date=July 13, 2017|archive-url=https://web.archive.org/web/20170713125418/http://ns.umich.edu/new/releases/22942-variations-in-atmospheric-oxygen-levels-shaped-earth-s-climate-through-the-ages|url-status=live}}</ref>

At the current rate of photosynthesis it would take about 2,000&nbsp;years to regenerate the entire {{chem|O|2}} in the present atmosphere.<ref>{{cite journal|title=The Natural History of Oxygen|first=Malcolm|last=Dole |journal=The Journal of General Physiology|volume=49|pages=5–27|date=1965|doi=10.1085/jgp.49.1.5|pmid=5859927|issue=1|pmc=2195461}}</ref>
{{clear}}

It is estimated that oxygen on Earth will last for about one billion years.<ref>{{Cite journal|url=https://www.nature.com/articles/s41561-021-00693-5|title=The future lifespan of Earth's oxygenated atmosphere|first1=Kazumi|last1=Ozaki|first2=Christopher T.|last2=Reinhard|date=March 9, 2021|journal=Nature Geoscience|volume=14|issue=3|pages=138–142|via=www.nature.com|doi=10.1038/s41561-021-00693-5|arxiv=2103.02694|bibcode=2021NatGe..14..138O |s2cid=232083548 }}</ref><ref>{{Cite web|url=https://www.eurekalert.org/news-releases/825455|title=How much longer will the oxygen-rich atmosphere be sustained on Earth?|website=EurekAlert!}}</ref>

===Extraterrestrial free oxygen===
{{Main|Extraterrestrial atmosphere}}
In the field of astrobiology and in the search for extraterrestrial life oxygen is a strong biosignature. That said it might not be a definite biosignature, being possibly produced abiotically on celestial bodies with processes and conditions (such as a peculiar hydrosphere) which allow free oxygen,<ref>{{cite web|url=https://earthsky.org/space/oxygen-exoplanets-not-always-indicator-of-life|title=Oxygen and life: a cautionary tale|date=3 January 2019|author=Paul Scott Anderson|access-date=29 December 2020|archive-date=January 22, 2021|archive-url=https://web.archive.org/web/20210122134654/https://earthsky.org/space/oxygen-exoplanets-not-always-indicator-of-life|url-status=live}}</ref><ref>{{cite journal | vauthors = Luger R, Barnes R | title = Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs | journal = Astrobiology | volume = 15 | issue = 2 | pages = 119–43 | date = February 2015 | pmid = 25629240 | pmc = 4323125 | doi = 10.1089/ast.2014.1231 | bibcode = 2015AsBio..15..119L | arxiv = 1411.7412 }}</ref><ref>{{cite journal |last1=Wordsworth |first1=Robin |last2=Pierrehumbert |first2=Raymond |title=Abiotic oxygen-dominated atmospheres on terrestrial habitable zone planets |journal=The Astrophysical Journal |date=1 April 2014 |volume=785 |issue=2 |pages=L20 |doi=10.1088/2041-8205/785/2/L20 |bibcode=2014ApJ...785L..20W |arxiv=1403.2713 |s2cid=17414970 }}</ref> like with Europa's and Ganymede's thin oxygen atmospheres.<ref name="Hall1998">{{cite journal |last1=Hall |first1=D.T. |last2=Feldman |first2=P.D. |last3=McGrath |first3=M.A. |last4=Strobel |first4=D. F. |display-authors=2 |title=The Far-Ultraviolet Oxygen Airglow of Europa and Ganymede |journal=The Astrophysical Journal |date=1998 |volume=499 |issue=1 |pages=475–81 |doi=10.1086/305604 |bibcode=1998ApJ...499..475H |doi-access=free }}</ref>

==Industrial production==
{{see also|Air separation|Oxygen evolution|Fractional distillation}}
[[File:Hofmann voltameter fr.svg|thumb|upright|Hofmann electrolysis apparatus used in electrolysis of water.|alt=A drawing of three vertical pipes connected at the bottom and filled with oxygen (left pipe), water (middle) and hydrogen (right). Anode and cathode electrodes are inserted into the left and right pipes and externally connected to a battery.]]

One hundred&nbsp;million&nbsp;tonnes of {{chem|O|2}} are extracted from air for industrial uses annually by two primary methods.<ref name="NBB300" /> The most common method is fractional distillation of liquefied air, with {{chem|N|2}} distilling as a vapor while {{chem|O|2}} is left as a liquid.<ref name="NBB300" />

The other primary method of producing {{chem|O|2}} is passing a stream of clean, dry air through one bed of a pair of identical zeolite molecular sieves, which absorbs the nitrogen and delivers a gas stream that is 90%&nbsp;to&nbsp;93% {{chem|O|2}}.<ref name="NBB300" /> Simultaneously, nitrogen gas is released from the other nitrogen-saturated zeolite bed, by reducing the chamber operating pressure and diverting part of the oxygen gas from the producer bed through it, in the reverse direction of flow. After a set cycle time the operation of the two beds is interchanged, thereby allowing for a continuous supply of gaseous oxygen to be pumped through a pipeline. This is known as pressure swing adsorption. Oxygen gas is increasingly obtained by these non-cryogenic technologies (see also the related vacuum swing adsorption).<ref>{{cite web|url=http://www.uigi.com/noncryo.html|title=Non-Cryogenic Air Separation Processes|date=2003|access-date=December 16, 2007|publisher=UIG Inc.|archive-date=October 3, 2018|archive-url=https://web.archive.org/web/20181003082121/http://www.uigi.com/noncryo.html|url-status=live}}</ref>

Oxygen gas can also be produced through electrolysis of water into molecular oxygen and hydrogen. DC electricity must be used: if AC is used, the gases in each limb consist of hydrogen and oxygen in the explosive ratio 2:1. A similar method is the electrocatalytic {{chem|O|2}} evolution from oxides and oxoacids. Chemical catalysts can be used as well, such as in chemical oxygen generators or oxygen candles that are used as part of the life-support equipment on submarines, and are still part of standard equipment on commercial airliners in case of depressurization emergencies. Another air separation method is forcing air to dissolve through ceramic membranes based on zirconium dioxide by either high pressure or an electric current, to produce nearly pure {{chem|O|2}} gas.<ref name="NBB301" />

==Storage==
[[File:Compressed gas cylinders.mapp and oxygen.triddle.jpg|thumb|Oxygen and MAPP gas compressed-gas cylinders with regulators]]

Oxygen storage methods include high-pressure oxygen tanks, cryogenics and chemical compounds. For reasons of economy, oxygen is often transported in bulk as a liquid in specially insulated tankers, since one liter of liquefied oxygen is equivalent to 840&nbsp;liters of gaseous oxygen at atmospheric pressure and {{convert|20|C|F}}.<ref name="NBB300" /> Such tankers are used to refill bulk liquid-oxygen storage containers, which stand outside hospitals and other institutions that need large volumes of pure oxygen gas. Liquid oxygen is passed through heat exchangers, which convert the cryogenic liquid into gas before it enters the building. Oxygen is also stored and shipped in smaller cylinders containing the compressed gas; a form that is useful in certain portable medical applications and oxy-fuel welding and cutting.<ref name="NBB300" />

==Applications==
{{see also|Breathing gas|Redox|Combustion}}

===Medical===
[[File:Home oxygen concentrator.jpg|thumb|upright|left|An oxygen concentrator in an emphysema patient's house|alt=A gray device with a label DeVILBISS LT4000 and some text on the front panel. A green plastic pipe is running from the device.]]

{{Main|Oxygen therapy}}

Uptake of {{chem|O|2}} from the air is the essential purpose of respiration, so oxygen supplementation is used in medicine. Treatment not only increases oxygen levels in the patient's blood, but has the secondary effect of decreasing resistance to blood flow in many types of diseased lungs, easing work load on the heart. Oxygen therapy is used to treat emphysema, pneumonia, some heart disorders (congestive heart failure), some disorders that cause increased pulmonary artery pressure, and any disease that impairs the body's ability to take up and use gaseous oxygen.<ref name="ECE510">[[#Reference-idCook1968|Cook & Lauer 1968]], p. 510</ref>

Treatments are flexible enough to be used in hospitals, the patient's home, or increasingly by portable devices. Oxygen tents were once commonly used in oxygen supplementation, but have since been replaced mostly by the use of oxygen masks or nasal cannulas.<ref name="pmid18540928">{{cite journal |author=Sim MA |display-authors=4 |author2=Dean P |author3=Kinsella J |author4= Black R |author5=Carter R|author6=Hughes M |title=Performance of oxygen delivery devices when the breathing pattern of respiratory failure is simulated |journal=Anaesthesia |volume=63 |issue=9 |pages=938–40 |date=2008 |pmid=18540928 |doi=10.1111/j.1365-2044.2008.05536.x|s2cid=205248111 |doi-access=free }}</ref>

Hyperbaric (high-pressure) medicine uses special oxygen chambers to increase the partial pressure of {{chem|O|2}} around the patient and, when needed, the medical staff.<ref name="pmid8931286">{{cite journal |author=Stephenson RN |author2=Mackenzie I |author3=Watt SJ |author4=Ross JA |title=Measurement of oxygen concentration in delivery systems used for hyperbaric oxygen therapy |journal=Undersea Hyperb Med |volume=23 |issue=3 |pages=185–88 |date=1996 |pmid=8931286 |url=http://archive.rubicon-foundation.org/2245 |access-date=September 22, 2008 |archive-date=August 11, 2011 |archive-url=https://web.archive.org/web/20110811175247/http://archive.rubicon-foundation.org/2245 |url-status=usurped }}</ref> Carbon monoxide poisoning, gas gangrene, and decompression sickness (the 'bends') are sometimes addressed with this therapy.<ref>{{cite web|url=http://www.uhms.org/Default.aspx?tabid=270 |title=Indications for hyperbaric oxygen therapy |author=Undersea and Hyperbaric Medical Society |access-date=September 22, 2008 |author-link=Undersea and Hyperbaric Medical Society |url-status=dead |archive-url=https://web.archive.org/web/20080912184905/http://www.uhms.org/Default.aspx?tabid=270 |archive-date=September 12, 2008 }}</ref> Increased {{chem|O|2}} concentration in the lungs helps to displace carbon monoxide from the heme group of hemoglobin.<ref>{{cite web |url=http://www.uhms.org/ResourceLibrary/Indications/CarbonMonoxidePoisoning/tabid/272/Default.aspx |title=Carbon Monoxide |author=Undersea and Hyperbaric Medical Society |access-date=September 22, 2008 |archive-url=https://web.archive.org/web/20080725005744/http://www.uhms.org/ResourceLibrary/Indications/CarbonMonoxidePoisoning/tabid/272/Default.aspx <!--Added by H3llBot--> |archive-date=July 25, 2008}}</ref><ref name="pmid15233173">{{cite journal |author=Piantadosi CA |title=Carbon monoxide poisoning |journal=Undersea Hyperb Med |volume=31 |issue=1 |pages=167–77 |date=2004 |pmid=15233173 |url=http://archive.rubicon-foundation.org/4002 |access-date=September 22, 2008 |archive-date=February 3, 2011 |archive-url=https://web.archive.org/web/20110203090807/http://archive.rubicon-foundation.org/4002 |url-status=usurped }}</ref> Oxygen gas is poisonous to the anaerobic bacteria that cause gas gangrene, so increasing its partial pressure helps kill them.<ref>{{cite journal |author=Hart GB |author2=Strauss MB |title=Gas Gangrene&nbsp;– Clostridial Myonecrosis: A Review |journal=J. Hyperbaric Med |volume=5 |issue=2 |pages=125–44 |date=1990 |url=http://archive.rubicon-foundation.org/4428 |access-date=September 22, 2008 |archive-date=February 3, 2011 |archive-url=https://web.archive.org/web/20110203090838/http://archive.rubicon-foundation.org/4428 |url-status=usurped }}</ref><ref>{{cite journal |author=Zamboni WA |author2=Riseman JA |author3=Kucan JO |title=Management of Fournier's Gangrene and the role of Hyperbaric Oxygen |journal=J. Hyperbaric Med |volume=5 |issue=3 |pages=177–86 |date=1990 |url=http://archive.rubicon-foundation.org/4431 |access-date=September 22, 2008 |archive-date=February 3, 2011 |archive-url=https://web.archive.org/web/20110203090958/http://archive.rubicon-foundation.org/4431 |url-status=usurped }}</ref> Decompression sickness occurs in divers who decompress too quickly after a dive, resulting in bubbles of inert gas, mostly nitrogen and helium, forming in the blood. Increasing the pressure of {{chem|O|2}} as soon as possible helps to redissolve the bubbles back into the blood so that these excess gasses can be exhaled naturally through the lungs.<ref name="ECE510" /><ref>{{cite web |url=http://www.uhms.org/ResourceLibrary/Indications/DecompressionSickness/tabid/275/Default.aspx |title=Decompression Sickness or Illness and Arterial Gas Embolism |author=Undersea and Hyperbaric Medical Society |access-date=September 22, 2008 |archive-url=https://web.archive.org/web/20080705210353/http://www.uhms.org/ResourceLibrary/Indications/DecompressionSickness/tabid/275/Default.aspx <!--Added by H3llBot--> |archive-date=July 5, 2008}}</ref><ref>{{cite journal |last=Acott |first=C. |title=A brief history of diving and decompression illness |journal=South Pacific Underwater Medicine Society Journal |volume=29 |issue=2 |date=1999 |url=http://archive.rubicon-foundation.org/6004 |access-date=September 22, 2008 |archive-date=September 5, 2011 |archive-url=https://web.archive.org/web/20110905152645/http://archive.rubicon-foundation.org/6004 |url-status=usurped }}</ref> Normobaric oxygen administration at the highest available concentration is frequently used as first aid for any diving injury that may involve inert gas bubble formation in the tissues. There is epidemiological support for its use from a statistical study of cases recorded in a long term database.<ref name="Longphre et al 2007">{{cite journal|title=First aid normobaric oxygen for the treatment of recreational diving injuries |last1=Longphre |first1=JM |last2=Denoble |first2=PJ |last3=Moon |first3=RE |last4=Vann |first4=RD |last5=Freiberger |first5=JJ |journal=Undersea & Hyperbaric Medicine |date=2007 |volume=34 |issue=1 |pages=43–49|url=https://pdfs.semanticscholar.org/3c96/eec9b2ae3f25ffc0569f26b7329d5b05e213.pdf |archive-url=https://web.archive.org/web/20181001104203/https://pdfs.semanticscholar.org/3c96/eec9b2ae3f25ffc0569f26b7329d5b05e213.pdf |url-status=dead |archive-date=2018-10-01 |via=Rubicon Research Repository |pmid=17393938 |s2cid=3236557 }}</ref><ref name="Emergency O2 for scuba">{{cite web |url=https://www.diversalertnetwork.org/training/courses/course_eo2 |title=Emergency Oxygen for Scuba Diving Injuries |publisher=Divers Alert Network |author=<!--not specified--> |access-date=October 1, 2018 |archive-date=April 20, 2020 |archive-url=https://web.archive.org/web/20200420114653/https://www.diversalertnetwork.org/training/courses/course_eo2 |url-status=live }}</ref><ref name="DAN Europe">{{cite web |url=https://daneurope.org/web/guest/readarticle;jsessionid=F8EB8916CD93E6A793F9F875BF5FC782?p_p_id=web_content_reading&p_p_lifecycle=0&p_p_mode=view&p_r_p_-1523133153_groupId=10103&p_r_p_-1523133153_articleId=11601&p_r_p_-1523133153_articleVersion=1.0&p_r_p_-1523133153_commaCategories=&p_r_p_-1523133153_commaTags= |title=Oxygen First Aid for Scuba Diving Injuries |publisher=Divers Alert Network Europe |author=<!--not specified--> |access-date=October 1, 2018 |archive-date=June 10, 2020 |archive-url=https://web.archive.org/web/20200610202203/https://daneurope.org/web/guest/readarticle;jsessionid=F8EB8916CD93E6A793F9F875BF5FC782?p_p_id=web_content_reading&p_p_lifecycle=0&p_p_mode=view&p_r_p_-1523133153_groupId=10103&p_r_p_-1523133153_articleId=11601&p_r_p_-1523133153_articleVersion=1.0&p_r_p_-1523133153_commaCategories=&p_r_p_-1523133153_commaTags= |url-status=live }}</ref>
{{clear}}

===Life support and recreational use===
[[File:STS057-89-067 - Wisoff on the Arm (Retouched).jpg|thumb|Low-pressure pure {{chem|O|2}} is used in space suits.]]

An application of {{chem|O|2}} as a low-pressure breathing gas is in modern space suits, which surround their occupant's body with the breathing gas. These devices use nearly pure oxygen at about one-third normal pressure, resulting in a normal blood partial pressure of {{chem|O|2}}. This trade-off of higher oxygen concentration for lower pressure is needed to maintain suit flexibility.<ref name="pmid11541018">{{cite journal|author=Morgenthaler GW|author2=Fester DA|author3=Cooley CG| title=As assessment of habitat pressure, oxygen fraction, and EVA suit design for space operations|journal=Acta Astronautica |volume= 32|issue=1|pages=39–49|date=1994|pmid=11541018|doi=10.1016/0094-5765(94)90146-5|bibcode = 1994AcAau..32...39M }}</ref><ref name="pmid2730484">{{cite journal|author=Webb JT|author2= Olson RM|author3=Krutz RW|author4=Dixon G|author5=Barnicott PT|title=Human tolerance to 100% oxygen at 9.5 psia during five daily simulated 8-hour EVA exposures|journal=Aviat Space Environ Med|volume=60|issue=5|pages=415–21|date=1989|pmid=2730484|doi=10.4271/881071}}</ref>

Scuba and surface-supplied underwater divers and submarines also rely on artificially delivered {{chem|O|2}}. Submarines, submersibles and atmospheric diving suits usually operate at normal atmospheric pressure. Breathing air is scrubbed of carbon dioxide by chemical extraction and oxygen is replaced to maintain a constant partial pressure. Ambient pressure divers breathe air or gas mixtures with an oxygen fraction suited to the operating depth. Pure or nearly pure {{chem|O|2}} use in diving at pressures higher than atmospheric is usually limited to rebreathers, or decompression at relatively shallow depths (~6 meters depth, or less),<ref name="Acott">{{cite journal|last=Acott|first=C.|title=Oxygen toxicity: A brief history of oxygen in diving|journal=South Pacific Underwater Medicine Society Journal|volume=29|issue=3|date=1999|url=http://archive.rubicon-foundation.org/6014|access-date=September 21, 2008|archive-date=December 25, 2010|archive-url=https://web.archive.org/web/20101225073221/http://archive.rubicon-foundation.org/6014|url-status=usurped}}</ref><ref name="Longphre">{{cite journal|last1=Longphre|first1=J. M.|title=First aid normobaric oxygen for the treatment of recreational diving injuries|journal=Undersea Hyperb. Med.|volume=34|issue=1|pages=43–49|date=2007|pmid=17393938|url=http://archive.rubicon-foundation.org/5514|access-date=September 21, 2008|display-authors=4|last2=Denoble|first2=P. J.|last3=Moon|first3=R. E.|last4=Vann|first4=R. D.|last5=Freiberger|first5=J. J.|archive-url=https://web.archive.org/web/20080613163501/http://archive.rubicon-foundation.org/5514|archive-date=June 13, 2008|url-status=usurped}}</ref> or medical treatment in recompression chambers at pressures up to 2.8 bar, where acute oxygen toxicity can be managed without the risk of drowning. Deeper diving requires significant dilution of {{chem|O|2}} with other gases, such as nitrogen or helium, to prevent oxygen toxicity.<ref name="Acott" />

People who climb mountains or fly in non-pressurized fixed-wing aircraft sometimes have supplemental {{chem|O|2}} supplies.<ref group=lower-alpha>The reason is that increasing the proportion of oxygen in the breathing gas at low pressure acts to augment the inspired {{chem|O|2}} partial pressure nearer to that found at sea-level.</ref> Pressurized commercial airplanes have an emergency supply of {{chem|O|2}} automatically supplied to the passengers in case of cabin depressurization. Sudden cabin pressure loss activates chemical oxygen generators above each seat, causing oxygen masks to drop. Pulling on the masks "to start the flow of oxygen" as cabin safety instructions dictate, forces iron filings into the sodium chlorate inside the canister.<ref name="NBB301" /> A steady stream of oxygen gas is then produced by the exothermic reaction.

Oxygen, as a mild euphoric, has a history of recreational use in oxygen bars and in sports. Oxygen bars are establishments found in the United States since the late 1990s that offer higher than normal {{chem|O|2}} exposure for a minimal fee.<ref name="FDA-O2Bars">{{cite journal|url=https://www.fda.gov/Fdac/features/2002/602_air.html| title=Oxygen Bars: Is a Breath of Fresh Air Worth It?|last=Bren|first=Linda|journal=FDA Consumer Magazine| volume=36| issue=6| pages=9–11|publisher=U.S. Food and Drug Administration|date=November–December 2002|access-date=December 23, 2007|archive-url=https://web.archive.org/web/20071018041754/https://www.fda.gov/Fdac/features/2002/602_air.html|archive-date=October 18, 2007|url-status=dead| pmid=12523293}}</ref> Professional athletes, especially in American football, sometimes go off-field between plays to don oxygen masks to boost performance. The pharmacological effect is doubted; a placebo effect is a more likely explanation.<ref name="FDA-O2Bars" /> Available studies support a performance boost from oxygen enriched mixtures only if it is inhaled ''during'' aerobic exercise.<ref>{{cite web|url=http://www.pponline.co.uk/encyc/1008.htm|title= Ergogenic Aids|access-date=January 4, 2008|publisher=Peak Performance Online |archive-url = https://web.archive.org/web/20070928051412/http://www.pponline.co.uk/encyc/1008.htm <!--Added by H3llBot--> |archive-date = September 28, 2007}}</ref>

Other recreational uses that do not involve breathing include pyrotechnic applications, such as George Goble's five-second ignition of barbecue grills.<ref>{{cite web|url=http://www.bkinzel.de/misc/ghg/index.html|title=George Goble's extended home page (mirror)|access-date=March 14, 2008|archive-url=https://web.archive.org/web/20090211213613/http://www.bkinzel.de/misc/ghg/index.html|archive-date=February 11, 2009|url-status=dead}}</ref><!-- - Primary source; many secondary sources exist but they only provide less information and more ads - -->

===Industrial===
[[File:Clabecq JPG01.jpg|thumb|Most commercially produced {{chem|O|2}} is used to smelt and/or decarburize iron.|alt=An elderly worker in a helmet is facing his side to the viewer in an industrial hall. The hall is dark but is illuminated yellow glowing splashes of a melted substance.]]

Smelting of iron ore into steel consumes 55% of commercially produced oxygen.<ref name="NBB301" /> In this process, {{chem|O|2}} is injected through a high-pressure lance into molten iron, which removes sulfur impurities and excess carbon as the respective oxides, {{chem|SO|2}} and {{chem|CO|2}}. The reactions are exothermic, so the temperature increases to 1,700&nbsp;°C.<ref name="NBB301" />

Another 25% of commercially produced oxygen is used by the chemical industry.<ref name="NBB301" /> Ethylene is reacted with {{chem|O|2}} to create ethylene oxide, which, in turn, is converted into ethylene glycol; the primary feeder material used to manufacture a host of products, including antifreeze and polyester polymers (the precursors of many plastics and fabrics).<ref name="NBB301" />

Most of the remaining 20% of commercially produced oxygen is used in medical applications, metal cutting and welding, as an oxidizer in rocket fuel, and in water treatment.<ref name="NBB301" /> Oxygen is used in oxyacetylene welding, burning acetylene with {{chem|O|2}} to produce a very hot flame. In this process, metal up to {{convert|60|cm|abbr=on}} thick is first heated with a small oxy-acetylene flame and then quickly cut by a large stream of {{chem|O|2}}.<ref name="ECE508">[[#Reference-idCook1968|Cook & Lauer 1968]], p. 508</ref>

==Compounds==
{{Main|Oxygen compounds}}
<!-- DIRECT ALL FUTURE EXPANSION to Oxygen compounds -->
[[File:Stilles Mineralwasser.jpg|thumb|upright|Water ({{chem|H|2|O}}) is the most familiar oxygen compound.|alt=Water flowing from a bottle into a glass.]]
The oxidation state of oxygen is −2 in almost all known compounds of oxygen. The oxidation state −1 is found in a few compounds such as peroxides.<ref>{{Greenwood&Earnshaw}}, p. 28</ref> Compounds containing oxygen in other oxidation states are very uncommon: −1/2 (superoxides), −1/3 (ozonides), 0 (elemental, hypofluorous acid), +1/2 (dioxygenyl), +1 (dioxygen difluoride), and +2 (oxygen difluoride).<ref>IUPAC: [http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf ''Red Book.''] {{Webarchive|url=https://web.archive.org/web/20180709210050/http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf |date=July 9, 2018 }} pp.&nbsp;73, 320.</ref>

===Oxides and other inorganic compounds===
<!-- DIRECT ALL FUTURE EXPANSION to Compounds of oxygen -->
Water ({{chem|H|2|O}}) is an oxide of hydrogen and the most familiar oxygen compound. Hydrogen atoms are covalently bonded to oxygen in a water molecule but also have an additional attraction (about 23.3&nbsp;kJ/mol per hydrogen atom) to an adjacent oxygen atom in a separate molecule.<ref>{{cite journal|first1=P.|last1=Maksyutenko|first2=T. R.|last2=Rizzo|first3=O. V.|last3=Boyarkin|date=2006|title=A direct measurement of the dissociation energy of water|pmid=17115729|journal=J. Chem. Phys.|page=181101 |doi=10.1063/1.2387163|issue=18|volume=125|bibcode = 2006JChPh.125r1101M }}</ref> These hydrogen bonds between water molecules hold them approximately 15% closer than what would be expected in a simple liquid with just van der Waals forces.<ref>{{cite web|title=Water Hydrogen Bonding|last=Chaplin|first=Martin|url=http://www.lsbu.ac.uk/water/hbond.html|access-date=January 6, 2008|date=January 4, 2008|archive-date=October 10, 2007|archive-url=https://web.archive.org/web/20071010055658/http://www.lsbu.ac.uk/water/hbond.html|url-status=live}}</ref><ref group=lower-alpha>Also, since oxygen has a higher electronegativity than hydrogen, the charge difference makes it a polar molecule. The interactions between the different dipoles of each molecule cause a net attraction force.</ref>

[[File:Rust screw.jpg|thumb|left|Oxides, such as iron oxide or rust, form when oxygen combines with other elements.|alt=A rusty piece of a bolt.]]
Due to its electronegativity, oxygen forms chemical bonds with almost all other elements to give corresponding oxides. The surface of most metals, such as aluminium and titanium, are oxidized in the presence of air and become coated with a thin film of oxide that passivates the metal and slows further corrosion. Many oxides of the transition metals are non-stoichiometric compounds, with slightly less metal than the chemical formula would show. For example, the mineral FeO (wüstite) is written as <math chem>\ce{Fe}_{1-x}\ce{O}</math>, where ''x'' is usually around 0.05.<ref>{{cite book|first1=Lesley E.|last1=Smart|last2=Moore|first2=Elaine A. |title=Solid State Chemistry: An Introduction|edition=3rd |publisher=CRC Press|date=2005|page=214|isbn=978-0-7487-7516-3}}</ref>

Oxygen is present in the atmosphere in trace quantities in the form of carbon dioxide ({{chem|CO|2}}). The Earth's crustal rock is composed in large part of oxides of silicon (silica {{chem|SiO|2}}, as found in granite and quartz), aluminium (aluminium oxide {{chem|Al|2|O|3}}, in bauxite and corundum), iron (iron(III) oxide {{chem|Fe|2|O|3}}, in hematite and rust), and calcium carbonate (in limestone). The rest of the Earth's crust is also made of oxygen compounds, in particular various complex silicates (in silicate minerals). The Earth's mantle, of much larger mass than the crust, is largely composed of silicates of magnesium and iron.

Water-soluble silicates in the form of {{chem|Na|4|SiO|4}}, {{chem|Na|2|SiO|3}}, and {{chem|Na|2|Si|2|O|5}} are used as detergents and adhesives.<ref name="ECE507">[[#Reference-idCook1968|Cook & Lauer 1968]], p. 507</ref>

Oxygen also acts as a ligand for transition metals, forming transition metal dioxygen complexes, which feature metal–{{chem|O|2}}. This class of compounds includes the heme proteins hemoglobin and myoglobin.<ref>{{cite book|last=Crabtree|first=R.|title=The Organometallic Chemistry of the Transition Metals|edition=3rd |publisher=John Wiley & Sons|date=2001|page=152|isbn=978-0-471-18423-2}}</ref> An exotic and unusual reaction occurs with {{chem|PtF|6}}, which oxidizes oxygen to give O<sub>2</sub><sup>+</sup>PtF<sub>6</sub><sup>−</sup>, dioxygenyl hexafluoroplatinate.<ref name="ECE505">[[#Reference-idCook1968|Cook & Lauer 1968]], p.505</ref>

===Organic compounds===
<!-- DIRECT ALL FUTURE EXPANSION to Compounds of oxygen -->
[[File:Acetone-3D-vdW.png|thumb|Acetone is an important feeder material in the chemical industry.
{{legend|red|Oxygen}}
{{legend|black|Carbon}}
{{legend|white|Hydrogen|outline=silver}}
|alt=A ball structure of a molecule. Its backbone is a zig-zag chain of three carbon atoms connected in the center to an oxygen atom and on the end to 6 hydrogens.]]

Among the most important classes of organic compounds that contain oxygen are (where "R" is an organic group): alcohols (R-OH); ethers (R-O-R); ketones (R-CO-R); aldehydes (R-CO-H); carboxylic acids (R-COOH); esters (R-COO-R); acid anhydrides (R-CO-O-CO-R); and amides ({{chem|R-CO-NR|2}}). There are many important organic solvents that contain oxygen, including: acetone, methanol, ethanol, isopropanol, furan, THF, diethyl ether, dioxane, ethyl acetate, DMF, DMSO, acetic acid, and formic acid. Acetone ({{chem|(CH|3|)|2|CO}}) and phenol ({{chem|C|6|H|5|OH}}) are used as feeder materials in the synthesis of many different substances. Other important organic compounds that contain oxygen are: glycerol, formaldehyde, glutaraldehyde, citric acid, acetic anhydride, and acetamide. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms. The element is similarly found in almost all biomolecules that are important to (or generated by) life.

Oxygen reacts spontaneously with many organic compounds at or below room temperature in a process called autoxidation.<ref name="ECE506">[[#Reference-idCook1968|Cook & Lauer 1968]], p. 506</ref> Most of the organic compounds that contain oxygen are not made by direct action of {{chem|O|2}}. Organic compounds important in industry and commerce that are made by direct oxidation of a precursor include ethylene oxide and peracetic acid.<ref name="ECE507" />

==Safety and precautions==
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The NFPA 704 standard rates compressed oxygen gas as nonhazardous to health, nonflammable and nonreactive, but an oxidizer. Refrigerated liquid oxygen (LOX) is given a health hazard rating of 3 (for increased risk of hyperoxia from condensed vapors, and for hazards common to cryogenic liquids such as frostbite), and all other ratings are the same as the compressed gas form.<ref name="nfpa">{{cite web|url = http://www.rivcoeh.org/Portals/0/documents/guidance/hazmat/bep_nfparatings.pdf|publisher = Riverside County Department of Environmental Health|access-date = August 22, 2017|title = NFPA 704 ratings and id numbers for common hazardous materials|archive-date = July 11, 2019|archive-url = https://web.archive.org/web/20190711171240/http://www.rivcoeh.org/Portals/0/documents/guidance/hazmat/bep_nfparatings.pdf|url-status = live}}</ref>

===Toxicity===
{{Main|Oxygen toxicity}}
[[File:Symptoms of oxygen toxicity.png|thumb|left|upright=1.35|Main symptoms of oxygen toxicity<ref>{{cite journal |author=Dharmeshkumar N Patel |display-authors=4 |author2=Ashish Goel |author3=SB Agarwal |author4=Praveenkumar Garg |author5=Krishna K Lakhani |title=Oxygen Toxicity |journal=Indian Academy of Clinical Medicine |volume=4 |issue=3 |page=234 |date=2003 |url=http://medind.nic.in/jac/t03/i3/jact03i3p234.pdf |access-date=April 26, 2009 |archive-date=September 22, 2015 |archive-url=https://web.archive.org/web/20150922093352/http://medind.nic.in/jac/t03/i3/jact03i3p234.pdf |url-status=dead }}</ref>|alt=A diagram showing a male torso and listing symptoms of oxygen toxicity: Eyes&nbsp;– visual field loss, nearsightedness, cataract formation, bleeding, fibrosis; Head&nbsp;– seizures; Muscles&nbsp;– twitching; Respiratory system&nbsp;– jerky breathing, irritation, coughing, pain, shortness of breath, tracheobronchitis, acute respiratory distress syndrome.]]

Oxygen gas ({{chem|O|2}}) can be toxic at elevated partial pressures, leading to convulsions and other health problems.<ref name="Acott" /><ref group=lower-alpha>Since {{chem|O|2}}'s partial pressure is the fraction of {{chem|O|2}} times the total pressure, elevated partial pressures can occur either from high {{chem|O|2}} fraction in breathing gas or from high breathing gas pressure, or a combination of both.</ref><ref name="ECE511">[[#Reference-idCook1968|Cook & Lauer 1968]], p. 511</ref> Oxygen toxicity usually begins to occur at partial pressures more than 50 kilopascals&nbsp;(kPa), equal to about 50% oxygen composition at standard pressure or 2.5 times the normal sea-level {{chem|O|2}} partial pressure of about 21&nbsp;kPa. This is not a problem except for patients on mechanical ventilators, since gas supplied through oxygen masks in medical applications is typically composed of only 30–50% {{chem|O|2}} by volume (about 30&nbsp;kPa at standard pressure).<ref name="NBB299" />

At one time, premature babies were placed in incubators containing {{chem|O|2}}-rich air, but this practice was discontinued after some babies were blinded by the oxygen content being too high.<ref name="NBB299" />

Breathing pure {{chem|O|2}} in space applications, such as in some modern space suits, or in early spacecraft such as Apollo, causes no damage due to the low total pressures used.<ref name="pmid11541018" /><ref>{{cite web|last = Wade|first = Mark|date = 2007|url = http://www.astronautix.com/craftfam/spasuits.htm|title = Space Suits|publisher = Encyclopedia Astronautica |access-date=December 16, 2007 |url-status = dead|archive-url = https://web.archive.org/web/20071213122134/http://www.astronautix.com/craftfam/spasuits.htm |archive-date = December 13, 2007}}</ref> In the case of spacesuits, the {{chem|O|2}} partial pressure in the breathing gas is, in general, about 30&nbsp;kPa (1.4 times normal), and the resulting {{chem|O|2}} partial pressure in the astronaut's arterial blood is only marginally more than normal sea-level {{chem|O|2}} partial pressure.<ref>{{cite web |url=http://www.globalrph.com/martin_4_most2.htm |title=The Four Most Important Equations In Clinical Practice |last=Martin |first=Lawrence |website=GlobalRPh |publisher=David McAuley |access-date=June 19, 2013 |archive-date=September 5, 2018 |archive-url=https://web.archive.org/web/20180905215615/http://www.globalrph.com/martin_4_most2.htm |url-status=live }}</ref>

Oxygen toxicity to the lungs and central nervous system can also occur in deep scuba diving and surface-supplied diving.<ref name="NBB299" /><ref name="Acott" /> Prolonged breathing of an air mixture with an {{chem|O|2}} partial pressure more than 60&nbsp;kPa can eventually lead to permanent pulmonary fibrosis.<ref name="BMJ">{{cite journal |author=Wilmshurst P |title=Diving and oxygen |journal=BMJ |volume=317 |issue=7164 |pages=996–99 |date=1998 |pmid=9765173 |pmc=1114047 |doi=10.1136/bmj.317.7164.996}}</ref> Exposure to an {{chem|O|2}} partial pressure greater than 160&nbsp;kPa (about 1.6 atm) may lead to convulsions (normally fatal for divers). Acute oxygen toxicity (causing seizures, its most feared effect for divers) can occur by breathing an air mixture with 21% {{chem|O|2}} at {{convert|66|m|abbr=on}} or more of depth; the same thing can occur by breathing 100% {{chem|O|2}} at only {{convert|6|m|abbr=on}}.<ref name="BMJ" /><ref name="Donald">{{cite book |last=Donald |first=Kenneth |title=Oxygen and the Diver |isbn = 978-1-85421-176-7|date=1992 |publisher=SPA in conjunction with K. Donald |location=England}}</ref><ref name="Donald1">{{cite journal |author=Donald K. W. |title=Oxygen Poisoning in Man: Part I |journal=Br Med J |volume=1 |issue=4506 |pages=667–72 |date=1947 |pmc=2053251 |doi=10.1136/bmj.1.4506.667 |pmid=20248086}}</ref><ref name="Donald2">{{cite journal |author=Donald K. W. |title=Oxygen Poisoning in Man: Part II |journal=Br Med J |volume=1 |pages=712–17 |date=1947 |pmc=2053400|issue=4507 |doi=10.1136/bmj.1.4507.712 |pmid=20248096}}</ref>

===Combustion and other hazards===
[[File:Apollo 1 fire.jpg|thumb|right|The interior of the Apollo 1 Command Module. Pure {{chem|O|2}} at higher than normal pressure and a spark led to a fire and the loss of the Apollo 1 crew.|alt=The inside of a small spaceship, charred and apparently destroyed.]]
Highly concentrated sources of oxygen promote rapid combustion. Fire and explosion hazards exist when concentrated oxidants and fuels are brought into close proximity; an ignition event, such as heat or a spark, is needed to trigger combustion.<ref name="astm-tpt"/> Oxygen is the oxidant, not the fuel.

Concentrated {{chem|O|2}} will allow combustion to proceed rapidly and energetically.<ref name="astm-tpt" /> Steel pipes and storage vessels used to store and transmit both gaseous and liquid oxygen will act as a fuel; and therefore the design and manufacture of {{chem|O|2}} systems requires special training to ensure that ignition sources are minimized.<ref name="astm-tpt" /> The fire that killed the Apollo 1 crew in a launch pad test spread so rapidly because the capsule was pressurized with pure {{chem|O|2}} but at slightly more than atmospheric pressure, instead of the {{frac|1|3}} normal pressure that would be used in a mission.{{refn|No single ignition source of the fire was conclusively identified, although some evidence points to an arc from an electrical spark.<ref>Report of Apollo 204 Review Board NASA Historical Reference Collection, NASA History Office, NASA HQ, Washington, DC</ref>|group=lower-alpha}}<ref name="chiles">{{cite book|last=Chiles|first=James R.|date=2001|title=Inviting Disaster: Lessons from the edge of Technology: An inside look at catastrophes and why they happen|url=https://archive.org/details/invitingdisaster00jame|url-access=registration|location=New York|publisher=HarperCollins Publishers Inc.|isbn=978-0-06-662082-4}}</ref>

Liquid oxygen spills, if allowed to soak into organic matter, such as wood, petrochemicals, and asphalt can cause these materials to detonate unpredictably on subsequent mechanical impact.<ref name="astm-tpt" />
{{clear}}

==See also==
{{div col|colwidth=20em}}
* Geological history of oxygen
* Hypoxia (environmental) for {{chem|O|2}} depletion in aquatic ecology
* Ocean deoxygenation
* Hypoxia (medical), a lack of oxygen
* Limiting oxygen concentration
* Oxygen compounds
* Oxygen plant
* Oxygen sensor
* Dark oxygen
{{div col end}}
{{Subject bar
|book1=Oxygen
|book2=Period 2 elements
|book3=Chalcogens
|book4=Chemical elements (sorted&nbsp;alphabetically)
|book5=Chemical elements (sorted by number)
|portal1=Chemistry
|portal2=Medicine
|commons=y
|wikt=y
|v=y
|v-search=Oxygen atom
|b=y
|b-search=Wikijunior:The Elements/Oxygen
}}

==Notes==
{{reflist|30em|group=lower-alpha}}

==References==
<!-- Full reference information for Cook, Daintith, and Emsley given in the "General references" subsection -->
{{reflist | 30em}}

===General references===
<!-- Please do not list cite web references here unless it is cited more than once -->
* <!-- Co -->{{cite book| ref=Reference-idCook1968|title=The Encyclopedia of the Chemical Elements| chapter-url=https://archive.org/details/encyclopediaofch00hamp| chapter-url-access=registration|last1=Cook|first1=Gerhard A.|last2=Lauer|first2=Carol M.|publisher=Reinhold Book Corporation|location=New York|date=1968|pages=[https://archive.org/details/encyclopediaofch00hamp/page/499 499–512]|editor=Clifford A. Hampel|chapter=Oxygen| lccn=68-29938}}
* <!-- Em -->{{cite book|ref=Reference-idEmsley2001|title=Nature's Building Blocks: An A–Z Guide to the Elements|last=Emsley|first=John|publisher=Oxford University Press|date=2001|location=Oxford, England|isbn=978-0-19-850340-8|chapter=Oxygen|pages=[https://archive.org/details/naturesbuildingb0000emsl/page/297 297–304]|chapter-url=https://archive.org/details/naturesbuildingb0000emsl/page/297}}
* <!-- Ra -->{{cite book| ref=Reference-idRaven2005 |last1=Raven|first1=Peter H.|first2=Ray F.|last2=Evert|first3=Susan E.|last3=Eichhorn|title=Biology of Plants| url=https://archive.org/details/biologyofplants00rave_0 | url-access=registration |edition=7th|publisher=W. H. Freeman and Company Publishers|date=2005|location = New York|pages=[https://archive.org/details/biologyofplants00rave_0/page/115 115–27]|isbn = 978-0-7167-1007-3}}

==External links==
{{Spoken Wikipedia|En-oxygen-article.ogg|date=2008-06-23}}
* [https://www.periodicvideos.com/videos/008.htm Oxygen] at ''The Periodic Table of Videos'' (University of Nottingham)
* [https://www.organic-chemistry.org/chemicals/oxidations/oxygen.shtm Oxidizing Agents > Oxygen]
* [https://www.uigi.com/oxygen.html Oxygen (O<sub>2</sub>) Properties, Uses, Applications]
* [https://www.americanscientist.org/issues/pub/the-story-of-o Roald Hoffmann article on "The Story of O"]
* [https://www.webelements.com/webelements/elements/text/O/index.html WebElements.com&nbsp;– Oxygen]
* {{In Our Time|Oxygen|b0088nql|Oxygen}}
* [https://scrippso2.ucsd.edu/ Scripps Institute: Atmospheric Oxygen has been dropping for 20 years]

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