View source for Oxygen

{{Infobox element
|name = oxygen
|symbol = O
|left = [[nitrogen]]
|right = [[fluorine]]
|above = –
|below = [[sulfur|S]]
|atomic-number = 8
|atomic-weight = 15.9994(3)
|configuration = [He] 2s<sup>2</sup> 2p<sup>4</sup>
|phys-ref = &#32;(O<sub>2</sub>)<ref name="NIST-O2">{{NIST chemistry | name = Oxygen | id = 1S/O2/c1-2 | accessdate = 2010-03-15}}.</ref><ref name="AirLiquide">{{AirLiquide | name = Oxygen | id = 48 | accessdate = 2010-04-03}}.</ref>
|melting-point = 54.8(2) K (−218.8 °C)
|boiling-point = 90.2(2) K (−183.0 °C)
|critical-point = 154.58 K, 50.43 bar
|triple-point = 54.35 K, 1.52 mbar
|density = 1.354 kg m<sup>−3</sup> (1 atm, 15 °C)<br/>4.475 kg m<sup>−3</sup> (1 atm, 90.2&nbsp;K)<br/>1.141 g cm<sup>−3</sup> (l, 90.2&nbsp;K)
|chem-ref = <ref name="AirLiquide"/><ref>{{Allred (1961)}}.</ref>
|electronegativity = 3.44 (Pauling)
|solubility = 48.9 cm<sup>3</sup> dm<sup>−3</sup> (1 atm, 0&nbsp;°C)
|IE-ref = <ref name="NIST-Oat">{{NIST chemistry | name = Oxygen, atomic | id = 1S/O | accessdate = 2010-03-15}}.</ref><ref>{{CODATA 2002}}.</ref>
|IE1 = 13.618 06 eV,<br/>1313.943 kJ mol<sup>−1</sup>
|IE2 = 35.1211 eV,<br/>3388.67 kJ mol<sup>−1</sup>
|IE3 = 54.9355 eV,<br/>5300.47 kJ mol<sup>−1</sup>
|IE4 = 77.4135 eV,<br/>7469.27 kJ mol<sup>−1</sup>
|IE5 = 113.8989 eV,<br/>10 989.57 kJ mol<sup>−1</sup>
|IE6 = 138.1196 eV,<br/>13 326.52 kJ mol<sup>−1</sup>
|IE7 = 739.3268 eV,<br/>71 334.20 kJ mol<sup>−1</sup>
|IE8 = 871.4097 eV,<br/>84 078.26 kJ mol<sup>−1</sup>
|IE-total = 2043.8432 eV,<br/>197 200.9 kJ mol<sup>−1</sup>
|EA-ref = <ref>{{citation | last1 = Valli | first1 = Christophe | last2 = Blondel | first2 = Christophe | last3 = Delsart | first3 = Christian | title = Measuring electron affinities with the photodetachment microscope | journal = Phys. Rev. A | year = 1999 | volume = 59 | issue = 5 | pages = 3809–15 | doi = 10.1103/PhysRevA.59.3809}}.</ref>
|EA1 = 1.461 112(44) eV<br/>140.9759(42) kJ mol<sup>−1</sup>
|radius-ref = &#32;<ref>{{Cordero et al. (2008)}}.</ref><ref>{{Shannon (1976)}}.</ref><ref>{{Bondi (1964)}}.</ref>
|covalent-radius = 66(2) pm
|vdw-radius = 152 pm
|ionic-radius = 140 pm (O<sup>2−</sup>, ''O<sub>h</sub>'')
|thermo-ref = &#32;(O<sub>2</sub>)<ref name="NIST-O2"/><ref>{{CODATA thermo}}.</ref>
|entropy = 205.152(5) J K<sup>−1</sup> mol<sup>−1</sup>
|enthalpy-fusion = 0.444 kJ mol<sup>−1</sup>
|enthalpy-vaporization = 6.82 kJ mol<sup>−1</sup>
|enthalpy-atomization = 249.18(10) kJ mol<sup>−1</sup>
|entropy-atomization = −44.093(6) J K<sup>−1</sup> mol<sup>−1</sup>
|heat-capacity = 29.378 J K<sup>−1</sup> mol<sup>−1</sup>
|hazard-ref = <ref>{{CLP Regulation|index=008-001-00-8|page=361}}</ref>
|pictograms = {{GHS03|Ox. Gas 1}}{{GHS04|Press. Gas}}
|signal-word = DANGER
|hazard-statements = {{H-phrases|270}}
|misc-ref = 
|CAS-number = 7782-44-7 (O<sub>2</sub>)<br/>17778-80-2 (atomic)
|EC-number = 231-956-9
}}

'''Oxygen''' is a colourless gas which makes up about one fifth of the Earth's atmosphere.

==History==
The discovery of oxygen is often credited to English chemist [[Joseph Priestley]], although the full story is somewhat more involved. That there is a component of air which is necessary for combustion and respiration was recognized by [[Leonardo da Vinci]] in the fifteenth century,<ref name="G&E">{{Greenwood&Earnshaw1st|pages=698–756}}.</ref> and confirmed by English chemist [[John Mayow]] in the mid-seventeenth century.<ref>{{citation | contribution = De sal-nitro et spiritu nitro-aereo | title = Tractatus quinque medico-physici | last = Mayow | first = John | authorlink = John Mayow | year = 1674 | location = Oxford | publisher = Sheldonian}}, summarized in {{citation | journal = Phil. Trans. R. Soc. London | year = 1674 | volume = 9 | pages = 101–13 | url = http://gallica.bnf.fr/ark:/12148/bpt6k558143.image.f110.langEN}}.</ref> However, the interpretation of these results was hampered by the rise of [[phlogiston]] theory, which stated that substances gave off phlogiston during combustion: air that was no longer capable of supporting combustion was said to be saturated with phlogiston. The unequivocal identification of oxygen as a chemical substance would have to wait for its preparation by chemical means.

The preparation was first carried out by Swedish chemist [[Carl Wilhelm Scheele]] on several occasions during the period 1771–73. Scheele heated various compounds such as [[Potassium nitrate|KNO<sub>3</sub>]], [[Magnesium nitrate|Mg(NO<sub>3</sub>)<sub>2</sub>]] and [[Mercury(II) oxide|HgO]] and found that they gave off a gas that he named "vitriol air", which supported combustion better than normal air. Scheele's results, however, were not published until 1777.<ref>{{citation | first = Carl Wilhelm | last = Scheele | authorlink = Carl Wilhelm Scheele | title = Chemische Abhandlung von der Luft und dem Feuer | url = http://books.google.co.uk/books?id=beoTAAAAQAAJ | location = Uppsala and Leipzig | publisher = Siverderus | year = 1777}}; [http://web.lemoyne.edu/~giunta/scheele77.html Translated extracts].</ref> In the meantime, Priestley had isolated the gas given off by heating HgO and named it "dephlogisticated air", and published his results in 1775 after proving that the gas was different from [[nitrous oxide]].<ref>{{citation | title = Experiments and Observations on Different Kinds of Air. Volume II | edition = 2nd | year = 1775 | location = London | url = http://books.google.co.uk/books?id=gB0UAAAAQAAJ | first = Joseph | last = Priestley | authorlink = Joseph Priestley | publisher = J.&nbsp;Johnson}}.</ref>

Priestley's work certainly had the greater impact, as he was able to discuss it with French chemist [[Antoine Lavoisier]] in October 1774 during a visit to Paris with his mentor and employer the Earl of Shelbourne. Lavoisier repeated and extended Priestley's work, and published his results in 1777 as ''Mémoire sur la combustion en général''<ref>{{citation | first = Antoine | last = Lavoisier | authorlink = Antoine Lavoisier | title = Mémoire sur la combustion en général | url = http://www.lavoisier.cnrs.fr/ice/modules/ice2pdf/pdf/extraitPDF06-04-2010_9-53-25.pdf | journal = Mem. Acad. Sci. Paris | year = 1777 | pages = 592}}; {{citation | title = &OElig;uvres&nbsp;: Mémoires de chimie et de physique | location = Paris | publisher = Imprimerie impériale | year = 1862 | pages = 225–33}}.</ref> and ''Considérations générales sur la nature des acides''.<ref>{{citation | first = Antoine | last = Lavoisier | authorlink = Antoine Lavoisier | title = Considérations générales sur la nature des acides et sur les principes dont ils sont composés | url = http://books.google.co.uk/books?id=qbc-AAAAcAAJ&hl=en&pg=PA248#v=onepage | journal = Mem. Acad. Sci. Paris | year = 1778}}; {{citation | title = &OElig;uvres&nbsp;: Mémoires de chimie et de physique | location = Paris | publisher = Imprimerie impériale | year = 1862 | pages = 248–60}}.</ref> It was Lavoisier who proposed the name "oxygen" for the new gas, from the Greek ὀξύς (''oxys''; acid, literally "sharp", from the taste of acids) and -γενής (''-genēs''; producer, literally "begetter"), as he (incorrectly) believed that all [[acid]]s contained oxygen. In his monograph ''Réflexions sur le phlogistique'',<ref>{{citation | first = Antoine | last = Lavoisier | authorlink = Antoine Lavoisier | title = Réflexions sur le phlogistique pour servir de suite à la théorie de la combustion et de la calcination, publiée en 1777 | url = http://books.google.co.uk/books?id=qbc-AAAAcAAJ&hl=en&pg=PA623#v=onepage | journal = Mem. Acad. Sci. Paris | year = 1783 | page = 505}}; {{citation | title = &OElig;uvres&nbsp;: Mémoires de chimie et de physique | location = Paris | publisher = Imprimerie impériale | year = 1862 | pages = 623–55}}.</ref> published at some point between 1777 and 1783, Lavoisier comprehensively refuted the phlogiston theory.

==Occurance and production==
Oxygen is almost ubiquitous at the surface of the Earth. The approximate [[mass fraction]]s of oxygen are: crustal rocks 46%; the human body 61%; sea water 86%. The [[amount fraction]] of oxygen in the Earth's atmosphere (at sea level) is 20.9476%,<ref>{{citation | title = U.S. Standard Atmosphere, 1976 | url = http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539_1977009539.pdf | publisher = National Oceanic and Atmospheric Administration | location = Washington, D.C. | year = 1976 | page = 3}}.</ref> amounting to some 10<sup>15</sup>&nbsp;tonnes. Apart from the atmosphere, the vast majority of this oxygen is chemically combined in a wide variety of inorganic and organic compounds: in the atmosphere, oxygen exists almost entirely as [[dioxygen]] (O<sub>2</sub>) molecules, its normal elemental state, with a small (but very important) amount of [[ozone]] (O<sub>3</sub>).

While it is difficult to obtain precise statistics, oxygen is believed to be the third most important bulk industrial chemical, after [[sulfuric acid]] and [[lime]] but ahead of [[ammonia]] and [[nitrogen]], with production of at least 100&nbsp;million tonnes per year. It is produced by the [[fractional distillation]] of liquid air. As the transport costs for the raw material (air) are zero, it is often economic to locate the oxygen plant close to the consumer, even for relatively small consumers such as hospitals.

==Use==
The majority of oxygen produced (ca. 70%) is used in the [[basic oxygen process]] for [[steel]]making.<ref name="G&E"/> A stream of pure oxygen is blown into the molton [[iron]], where it burns off the residual [[carbon]] to produce low-carbon (0.1–1.0%) steel. Another large scale use is in [[oil refining]], particularly in [[fluid catalytic cracking]] (FCC) units that treat roughly one-third of the total [[crude oil]]. The oxygen does not play a direct role in the [[cracking]] process, but is used to regenerate the [[catalyst]], which rapidly gets contaminated with byproduct [[coke]]. The oxygen is mixed with air to give a mixture of around 28% O<sub>2</sub>,<ref name="AirLiquide"/> and this is used to burn the coke off the catalyst: the heat released from the combustion of the coke compensates for the heat lost in the endothermic cracking reactions, hence the need for a strict control of the rate of combustion.

Oxygen is also used in other bulk chemical processes, such as the manufactire of [[ethylene oxide]], [[propylene oxide]], [[ethylene dichloride]], [[vinyl chloride]], [[vinyl acetate]], [[titanium dioxide]] and [[ferric sulfate]]. It can also be used to enrich the air stream in processes that normally use air, such as the production of [[acrylonitrile]] and [[terephthalic acid]], and is used in the production of [[synthesis gas]] (CO/H<sub>2</sub>).<ref name="AirLiquide"/><ref name="G&E"/>

==Allotropes==
{{main|Dioxygen|Ozone}}
Oxygen forms two [[allotrope]]s: [[dioxygen]] (O<sub>2</sub>) and [[ozone]] (O<sub>3</sub>). Dioxygen is by far the commonest form. It is a colourless and odourless gas at ambient temperature and pressure, and condenses to a pale blue liquid at −183.0&nbsp;°C. Unusually for a diatomic molecule, it is [[Paramagnetism|paramagnetic]], with two unpaired electrons occupying π* orbitals.

Ozone (trioxygen) is an unstable, toxic gas with a characteristic odour. It is formed from dioxygen by the action of an electric discharge (e.g., during lighteneing storms) or [[ultraviolet light]] (e.g., in photochemical smog). The O<sub>3</sub> molecule is bent, with an angle of 116.8(5)° at the central oxygen and bond lengths of 127.8(3)&nbsp;pm.

==Chemical properties==
===Water and hydrogen peroxide===
{{main|Water|Hydrogen peroxide}}

===Oxides===
{{main|Oxide}}
Oxygen forms binary compounds with all the elements except the [[noble gas]]es (although [[xenon]] forms two well-defined oxides). Oxides cover the full range of bonding types from almost completely [[Ionic bonding|ionic]] (e.g., [[caesium oxide]]) to almost completely [[Covalent bonding|covalent]] (e.g., [[chlorine dioxide]]), and show a huge variety of structural types.

A common classification system for oxides uses their [[Acid–base reaction|acid–base reactivity]]:
{| class="wikitable"
|-
! Type
! Characteristics
! Bonding
! Structure
! Examples
|-
| [[Basic oxide]]s
| react with acids
| predominently ionic
| classical ionc structures
| [[Sodium oxide|Na<sub>2</sub>O]], [[Calcium oxide|CaO]]
|-
| [[Amphoteric oxide]]s
| react with both acids and bases
| intermediate
| complex infinite structures
| [[Aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]]
|-
| [[Acidic oxide]]s
| react with bases
| predominently covalent
| molecular
| [[Phosphorus pentoxide|P<sub>2</sub>O<sub>5</sub>]], [[Sulfur dioxide|SO<sub>2</sub>]]
|-
| [[Neutral oxide]]s
| unreactive in acid–base reactions
| predominently covalent
| molecular
| [[Carbon monoxide|CO]], [[Nitric oxide|NO]]
|-
|}

====Oxygen fluorides====
{{main|Oxygen fluorides}}
As [[fluorine]] is more [[Electronegativity|electronegative]] than oxygen, their binary compounds are properly referred to as [[oxygen fluorides]] rather than fluorine oxides. The most stable is [[oxygen difluoride]], OF<sub>2</sub>, first prepared in 1929, a colourless, toxic gas (bp = −145.3&nbsp;°C) that is a powerful oxidizing and fluorinating agent. [[Dioxygen difluoride]], O<sub>2</sub>F<sub>2</sub>, and other unstable species are also known.<ref name="G&E"/>

===Dioxygen as a ligand===
{{main|Dioxygen complexes}}

===Organic compounds of oxygen===

==Biological role==

==Safety==

==Notes and references==
===Notes===
{{reflist|group=note}}

===References===
{{reflist|2}}

==External links==
{{wikipedia|Oxygen}}
*[http://www.webelements.com/oxygen/ WebElements]

[[Category:Chemical elements]]
[[Category:Chalcogens]]
[[Category:Oxygen|*]]

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