Editing Oxygen (section)

==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 principle|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 [[chemical bond|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 orbital]]s 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 principle|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 orbitals|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–particle duality|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 [[paramagnetism|paramagnetic]]. That is, they impart magnetic character to oxygen when it is in the presence of a magnetic field, because of the [[Spin (physics)|spin]] [[magnetic moment]]s 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 [[magnet]]ic 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 compound|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> [[Carotenoid]]s 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 [[Allotropy|allotrope]] of elemental oxygen on Earth is called [[Allotropes of oxygen|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;[[Picometre|pm]] and a bond energy of 498&nbsp;[[joule per mole|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]].

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 [[Electromagnetic spectrum|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 [[air pollution|pollutant]] formed as a by-product of [[exhaust system|automobile exhaust]].<ref name="GuideElem48" /> At [[low earth orbit]] altitudes, sufficient atomic oxygen is present to cause [[corrosion in space|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 [[Metastability in molecules|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;[[Pascal (unit)|GPa]], is in fact a [[rhombohedral]] {{chem|O|8}} [[Cluster chemistry|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 [[oxidizing agent|oxidizer]] than either {{chem|O|2}} or {{chem|O|3}} and may therefore be used in [[Rocket propellant|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 [[superconductivity|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 [[Solubility|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=6}} of air, freshwater can dissolve about 6.04&nbsp;[[Litre|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.{{cn|date=May 2025}}
{| 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;[[kelvin|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 oxygen|liquid]] and [[solid oxygen|solid]] {{chem|O|2}} are clear substances with a light [[diffuse sky radiation|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<sup>3</sup>)
|[[Specific heat]] (kJ/(kg·K))
|[[Dynamic viscosity]] (kg/(m·s))
|[[Kinematic viscosity]] (m<sup>2</sup>/s)
|[[Thermal conductivity]] (W/(m·K))
|[[Thermal diffusivity]] (m<sup>2</sup>/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 [[isotope]]s, [[oxygen-16|<sup>16</sup>O]], [[oxygen-17|<sup>17</sup>O]], and [[oxygen-18|<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 [[nucleosynthesis|synthesized]] at the end of the [[helium fusion]] process in massive [[star]]s 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 [[Nitrogen-14|<sup>14</sup>N]] (made abundant from CNO burning) captures a [[Helium-4|<sup>4</sup>He]] nucleus, making <sup>18</sup>O common in the helium-rich zones of [[Stellar evolution#Massive stars|evolved, massive stars]].<ref name="Meyer2005" />

Fifteen [[radioisotope]]s 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 decay|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 [[positron emission|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>
|-
![[Atomic number|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 [[crust (geology)#Earth's crust and mantle|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.<ref>{{cite book |author1=Richard Peer Wayne |title=Chemistry of Atmospheres |date=2006 |publisher=Oxford University Press |isbn=9780198503750 |language=en |pages=562–584}}</ref>

[[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 [[Cellular respiration|respiration]], [[Decomposition|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 pollution|Water polluted]] with plant nutrients such as [[nitrate]]s or [[phosphate]]s 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 variability and change|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.]]

[[Paleoclimatology|Paleoclimatologists]] measure the ratio of oxygen-18 and oxygen-16 in the [[Exoskeleton|shells]] and [[skeleton]]s 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.{{cn|date=May 2025}}

[[Geology of solar terrestrial planets|Planetary geologists]] have measured the relative quantities of oxygen isotopes in samples from the [[Earth]], the [[Moon]], [[Mars]], and [[meteorite]]s, but were long unable to obtain reference values for the isotope ratios in the [[Sun]], believed to be the same as those of the [[Nebular hypothesis|primordial solar nebula]]. Analysis of a [[silicon]] wafer exposed to the [[solar wind]] in space and returned by the crashed [[Genesis (spacecraft)|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 [[Protoplanetary disk|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 band]]s peaking at the wavelengths 687 and 760&nbsp;[[Nanometre|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 [[Earth observation satellite|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.{{cn|date=May 2025}}
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