Editing Oxygen (section)

===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>