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{{Infobox element
|name = helium
|symbol = He
|left = [[hydrogen]]
|right = [[lithium]]
|above = –
|below = [[Neon|Ne]]
|atomic-number = 2
|atomic-weight = 4.002 602(2)
|configuration = 1s<sup>2</sup>
|phys-ref = <ref name="NIST">{{NIST chemistry | name = Helium | id = 1S/He | accessdate = 2010-03-19}}.</ref>
|melting-point = ''see text''
|boiling-point = 4.22 K (−268.93&nbsp;°C)
|critical point = 5.2 K, 2.274 bar
|solubility = 8.61 cm<sup>3</sup> kg<sup>−1</sup> (101.325&nbsp;kPa, 20&nbsp;°C)
|IE-ref = <ref>{{RubberBible62nd|page=E-65}}.</ref>
|IE1 = 24.587 eV<br/>2372.3 kJ mol<sup>−1</sup>
|IE2 = 54.416 eV<br/>5250.5 kJ mol<sup>−1</sup>
|thermo-ref = <ref name="NIST"/><ref name="G&E">{{Greenwood&Earnshaw1st|pages=1042–59}}.</ref>
|entropy = 126.153(2) J K<sup>−1</sup> mol<sup>−1</sup>
|enthalpy-vaporization = 0.08 kJ mol<sup>−1</sup>
|CAS-number = 7440-59-7
}}
'''Helium''' (symbol: '''He''') is a [[chemical element]], the lightest of the [[noble gas]]es.

==History==
Helium is the only element to have been discovered extraterrestrially before being found on Earth, specifically in the Sun.<ref name="G&E"/> A bright yellow spectral line (''λ''&nbsp;= 587.49&nbsp;nm) was first observed by French astronomer [[Jules Janssen]] during the eclipse of 18&nbsp;August 1868, which Janssen observed from Guntur in India, and independently by [[Norman Lockyer]] in London on 20&nbsp;October 1868.<ref>{{citation | title = Sir Norman Lockyer, 1836–1920 | url = http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1921ApJ....53..233C&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf | last = Cortie | first = A. L. | journal = Astrophys. J. | year = 1921 | volume = 53 | issue = 4 | pages = 233–48}}.</ref><ref>{{citation | url = http://www.wired.com/thisdayintech/2009/08/dayintech_0818/  | publisher = wired.com | accessdate = 2010-03-18 | title = Aug. 18, 1868: Helium Discovered During Total Solar Eclipse | date = August 18, 2009 | first = Hadley | last = Leggett}}.</ref><ref>{{citation | journal = C. R. Hebd. Acad. Sci. Paris | volume = 67 | year = 1868 | pages = 836–41 | url = http://gallica.bnf.fr/ark:/12148/bpt6k3024c.image.r=comptes-rendus+hebdomadaires+Acad%C3%A9mie+des+Sciences.f836.langFR}}.</ref> Lockyer, together with English chemist [[Edward Frankland]], showed that the line could not be explained by any known element, and proposed the name helium, from the Greek {{Polytonic|ἥλιος}} (''helios''; the Sun).<ref>{{citation | contribution = Helium | title = Oxford English Dictionary | year = 2008 | url = http://dictionary.oed.com/cgi/entry/50104457? | publisher = Oxford University Press | accessdate = 2008-07-20}}.</ref><ref>"Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium." {{citation| last = Thomson | first = W. | year = 1872 | journal = Rep. Brit. Assoc. | page = 99}}.</ref>

The first isolation of helium from a terrestrial source is usually credited to the British chemist [[William Ramsey]], who isolated the helium that was occluded in the mineral [[cleveite]] (a [[uranium]]-containing mineral) and identified it by its spectrum.<ref>{{citation | title = On a Gas Showing the Spectrum of Helium, the Reputed Cause of D<sub>3</sub>, One of the Lines in the Coronal Spectrum. Preliminary Note | last = Ramsay | first = William | authorlink = William Ramsay | journal = Proc. Roy. Soc. London | volume = 58 | pages = 65–67 | year = 1895 | doi = 10.1098/rspl.1895.0006}}. {{citation | title = Helium, a Gaseous Constituent of Certain Minerals. Part I | last = Ramsay | first = William | authorlink = William Ramsay | journal = Proc. Roy. Soc. London | volume = 58 | pages = 80–89 | year = 1895 | doi = 10.1098/rspl.1895.0010}}. {{citation | title = Helium, a Gaseous Constituent of Certain Minerals. Part II | last = Ramsay | first = William | authorlink = William Ramsay | journal = Proc. Roy. Soc. London | volume = 59 | pages = 325–30 | year = 1895 | doi = 10.1098/rspl.1895.0097}}.</ref> The occluded gas had been noticed previously by American geochemist [[William Francis Hillebrand]], but had been misidentified as nitrogen.<ref>{{citation | year = 1999 | contribution = W. F. Hillebrand (1853–1925), geochemist and US Bureau of Standards administrator | title = American National Biography | editor1-first = John A. | editor1-last = Garraty | editor2-first = Mark C. | editor2-last = Carnes | volume = 10-11 | publisher=Oxford University Press | pages = 227–28, 808–9}}.</ref> Helium had been identified (through its spectrum) on Earth as early as 1881 by Italian physicist [[Luigi Palmieri]],<ref name="G&E"/> but Palmieri could not isolate a sample of the element. Finally, helium was isolated independently in 1895 by [[Per Teodor Cleve]]  and [[Abraham Langlet]]  in [[Uppsala]], [[Sweden]], who collected enough of the gas to accurately determine its [[atomic weight]].<ref>{{citation | title = Das Atomgewicht des Heliums | last = Langlet | first = N. A. | authorlink = Abraham Langlet | journal = Z. Anorg. Chem. | volume = 10 | issue = 1 | pages = 289–92 | year = 1895 | doi =10.1002/zaac.18950100130}}.</ref> Nevertheless, it was Ramsey who was awarded the [[Nobel Prize in Chemistry]] (in 1904),<ref>{{citation | title = The Nobel Prize in Chemistry 1904 | url = http://nobelprize.org/nobel_prizes/chemistry/laureates/1904/index.html | publisher = Nobel Foundation | accessdate = 2010-04-02}}.</ref> and who is usually credited with the discovery.

The first significant quantities of helium on Earth were discovered in the [[natural gas]] from a well in Kansas, USA, in 1903–6:<ref>{{citation | last = McFarland | first = D. F. | title = Composition of Gas from a Well at Dexter, Kan. | volume = 19 | pages = 60–62 | | year = 1903 | journal = Trans. Kansas Acad. Sci. | doi = 10.2307/3624173}}.</ref><ref>{{citation | last1 = Cady | first1 = H.P. | last2 = McFarland | first2 = D. F. | title = Helium in Natural Gas | journal = Science | volume = 24 | page = 344 | doi = 10.1126/science.24.611.344 | year = 1906}}.</ref> the discovery was designated a [[National Historic Chemical Landmark]] by the [[American Chemical Society]] in 2000.<ref>{{citation | title = The Discovery of Helium in Natural Gas | url = http://acswebcontent.acs.org/landmarks/landmarks/helium/helium.html | publisher = American Chemical Society | accessdate = 2010-04-02}}.</ref>

==Occurrence and production==
{| class="wikitable" align=right style="margin:0 0 0 0.5em; width:14em;"
|-
! Country
! <u>Production</u><br/>10<sup>6</sup>&nbsp;m<sup>3</sup><br/><ref group="note" name="m3">Most U.S. Geological Survey sources quote amounts of helium in millions of cubic metres, measured at 101.325&nbsp;kPa and 15&nbsp;°C. Older U.S. sources may quote values in cubic feet, measured at 14.7&nbsp;psi and 70&nbsp;°F: one cubic metre (101.325&nbsp;kPa, 15&nbsp;°C) = 36.053 cubic feet (14.7&nbsp;psi, 70&nbsp;°F). 10<sup>6</sup>&nbsp;m<sup>3</sup> (101.325&nbsp;kPa, 15&nbsp;°C) = 169.29&nbsp;tonnes.</ref>
|-
| [[United States]]
| align=right | 122*
|-
| [[Algeria]]
| align=right | 24
|-
| [[Qatar]]
| align=right | 15
|-
| [[Russia]]
| align=right | 7
|-
| [[Poland]]
| align=right | 2.5
|-
| colspan=2 | Estimates for 2009 from the<br/>U.S. Geological Survey<ref name="USGS-MCS">{{citation | first1 = Norbert | last1 = Pacheco | first2 = Diedre S. | last2 = Thomas | url = http://minerals.usgs.gov/minerals/pubs/commodity/helium/mcs-2010-heliu.pdf | title = Mineral Commodities Summaries | contribution = Helium | date = January 2010 | publisher = U.S. Geological Survey}}.</ref>
|-
| colspan=2 | <small>*U.S. figure includes 42 million<br/>cubic metres withdrawn from the<br/>federal government stockpile.</small>
|-
|}
Helium is the second most common element in the Universe (after [[hydrogen]]), accounting for 23% of all atoms. However, the Earth's gravitational field is not strong enough to retain helium in the atmosphere for long periods, and almost all the Earth's primordial helium is believed to have escaped.<ref>{{IUPAC atomic weight review 2000}}.</ref> The helium currently present on Earth has been formed from the [[α-decay]] of radioactive nuclides:<ref group="note">The [[helium-3]] isotope (''x''&nbsp;= 1.34(3)&nbsp;ppm in the atmosphere) is formed from the [[β-decay]] of [[tritium]] ({{Nuclide|Z=1|A=3}}): the existence of helium from volcanic rocks or associated geothermal springs and gases with ''x''(<sup>3</sup>He)&nbsp;> 10&nbsp;ppm indicates that at least some of this helium-3 is primordial.</ref> most of this helium escapes to the atmosphere, where the current [[volume fraction]] is 5.24&nbsp;ppm, and then into space: some of it, however, can be trapped underground by impermeable rock formations, often associated with [[natural gas]] deposits.

The commercial production of helium is based around its extraction from natural gas, which is economically viable when the helium fraction is greater than about 0.3%.<ref name="G&E"/> The [[United States]] has historically been the predominant producer, although its ''de novo'' production is declining and [[Algeria]] and [[Qatar]] are gaining importance.

The U.S. Bureau of Land Management operates a stockpile of crude (~80%) helium, the [[National Helium Reserve]] at Cliffside Field, Potter County, Texas, and a crude-helium pipeline from Bushton, Kansas, passing through the Reichel Field (Kansas) and the Keyes Field (Oklahoma) to the Cliffside Field.<ref name="MYB">{{citation | first = Norbert | last = Pacheco | url = http://minerals.usgs.gov/minerals/pubs/commodity/helium/myb1-2008-heliu.pdf | contribution = Helium | title = 2008 Minerals Yearbook | publisher = U.S. Geological Survey | date = October 2009}}.</ref> The National Helium Reserve reached its peak in 1980, with 194,000&nbsp;tonnes (1150&nbsp;million cubic metres) stored,<ref name="UGS-DS">{{citation | publisher = U.S. Geological Survey | year = 2008 | contribution = Helium statistics | last1 = Kelly | first1 = T. D. | last2 = Matos | first2 = G. R. | title = Historical statistics for mineral and material commodities in the United States | id = U.S. Geological Survey Data Series 140 | url = http://minerals.usgs.gov/ds/2005/140/helium.pdf | accessdate = 2010-03-19}}.</ref> but it is being run down under the terms of the [[Helium Privatization Act of 1996]]  (Pub.&nbsp;L. 104–273), and will eventually be just 600&nbsp;million cubic feet (16.6&nbsp;million cubic metres). About one third of the helium produced in the United States is refined from stored helium rather than being extracted from natural gas,<ref name="USGS-MCS"/> and the National Helium Reserve held 561&nbsp;million cubic metres of crude helium at the end of 2008.<ref name="MYB"/>

The refinement of crude helium to the normal commercial grade ("Grade&nbsp;I", about 99.995%) requires cooling with liquid [[hydrogen]] to below 27&nbsp;K (−246&nbsp;°C, −411&nbsp;°F) to condense out the [[neon]] present: liquid hydrogen is required, as helium gas cannot be cooled by the [[Joule–Thomson effect]] until its temperature is below about 40&nbsp;K.<ref>{{citation | first = J. | last = Ashmead | year = 1950 | | title = A Joule–Thomson Cascade Liquefier for Helium  | journal = Proc. Phys. Soc. B | volume = 63 | issue = 7 | pages = 504 | doi = 10.1088/0370-1301/63/7/304}}.</ref><ref name="VanSciver">{{citation | title = Helium cryogenics | first = Steven W. | last = Van Sciver | url = http://books.google.co.uk/books?id=bdYsrq2n5YYC | pages = 286–88 | publisher = Springer | year = 1986 | isbn = 0306423359}}.</ref> Commercial helium is usually shipped in bulk as a liquid.<ref name="MYB"/>

==Use==
{| class="wikitable"  align=right style="margin:0 0 0 0.5em; width:18em;"
|-
! colspan=2 | Total U.S. consumpton (2009)<br/>52.1 million cubic metres
|-
| Cryogenics
| align=right | 32%
|-
| Pressurizing and purging
| align=right | 18%
|-
| Controlled atmospheres
| align=right | 18%
|-
| Welding cover gas
| align=right | 13%
|-
| Leak detection
| align=right | 4%
|-
| Breathing mixtures
| align=right | 2%
|-
| Other uses
| align=right | 13%
|-
| colspan=2 | Estimates for 2009 from the U.S. Geological Survey<ref name="USGS-MCS"/><br/>The breakdown of usage in other countries may be significantly different: ''see text''.
|-
|}
The main use of helium is in cryogenics, where helium is essential for temperatures below −256&nbsp;°C (−429&nbsp;°F),<ref name="USGS-MCS"/> the approximate boiling point of liquid [[hydrogen]]. Cooling with liquid helium allows an operating temperature of around −269&nbsp;°C (−452&nbsp;°F), just four [[kelvin]]s. Such low temperatures are required for [[superconducting magnet]]s to operate; these are used in a variety of applications, including [[NMR spectrometer]]s and [[magnetic resonance imaging]] (MRI) scanners.

Uses for pressurizing and purging, in controlled atmospheres (e.g., [[glove box]]es) and as a welding cover gas are only important in the United States, where helium is relatively cheap, and are declining even there along with a general fall in U.S. helium consumption (−42% over the period 2000–2009).<ref name="UGS-DS"/><ref>{{citation | publisher = U.S. Geological Survey | year = 2003 | contribution = Helium statistics | last1 = Kelly | first1 = T. D. | last2 = Matos | first2 = G. R. | title = Historical statistics for mineral and material commodities in the United States | id = U.S. Geological Survey Data Series 140 | url = http://minerals.usgs.gov/ds/2005/140/helium-use.pdf | accessdate = 2010-03-19}}.</ref><ref group="note">U.S. consumption of helium in 2000 was 89.8 million cubic metres, of which: cryogenics 24%, pressurizing and purging 20%, welding cover gas 18%, controlled atmospheres 16%, leak detection 6%, breathing mixtures 3%, other uses (chromatography/lifting gas/heat transfer) 13%.</ref> In other countries, [[argon]] is used for these purposes.

There are several niche uses of helium, including:<ref name="G&E"/>
*as a means of detecting small leaks of gas in otherwise closed systems using the [[helium mass spectrometer]] approach, usually for leaks of {{nowrap|10<sup>−5</sup>–10<sup>−13</sup>&nbsp;Pa m<sup>3</sup> s<sup>−1</sup>}} (from about 1&nbsp;ml/min to less than 3&nbsp;ml per century)
*as a replacement for [[nitrogen]] in [[breathing mixture]]s which are to be delivered to humans at high pressure, such as in deep-sea diving, and also in some medical uses;
*as a flow gas in [[gas chromatography]], particularly when using a [[katharometer]], a [[helium ionization detector]] (HID) or in [[GC–MS]];
*as a lifting gas for meteorological balloons and airships;
*as a heat transfer agent, particularly in [[Very high temperature reactor|VHTR]] [[nuclear reactor]]s.

==Physical properties==
Helium has a number of unique physical properties. It is the [[chemical substance]] with the lowest [[boiling point]] (4.22&nbsp;K) and the only substance that cannot be solidified by cooling alone at atmospheric pressure. It is also the only substance without a [[triple point]], a temperature and pressure at which solid, liquid and gas are all in equilibrium.<ref name="G&E"/>

Helium has an extremely high [[specific heat capacity]] (''C<sub>p</sub>''&nbsp;= {{nowrap|5.19&nbsp;kJ K<sup>−1</sup> kg<sup>−1</sup>}} at 25&nbsp;°C)<ref>{{RubberBible62nd|page=D-155}}.</ref> and, for a gas, a relatively high [[thermal conductivity]] ({{nowrap|0.1430&nbsp;W m<sup>−1</sup> K<sup>−1</sup>}} at 0&nbsp;°C):<ref name="G&E"/> both of these are due to the low mass of the atoms which make up helium gas (H<sub>2</sub> is even more exceptional on both counts), and are important in its uses in [[gas chromatography]] and as a heat transfer agent.

Helium at low pressures and ambient temperatures is the nearest practical model to an [[ideal gas]], one in which the attractive forces between atoms are negligible, as is the volume of the atoms compared with the volume occupied by the gas. It has an extremely low [[Joule–Thomson inversion temperature]] of about 40–45&nbsp;K:<ref name="VanSciver"/><ref>{{citation | journal = Phys. Rev. | volume = 43 | issue = 1 | pages = 60–69 | year = 1933 | title = The Joule-Thomson Effect in Helium | first1 = J. R. | last1 = Roebuck | first2 = H. | last2 = Osterberg | doi = 10.1103/PhysRev.43.60}}.</ref> above this temperature, the Joule–Thomson coefficient is negative and so helium will warm up as it expands.

===Liquid helium===
{| class="wikitable" align=right style="margin:0 0 0 0.5em;"
|-
! rowspan=2 | &nbsp;
! colspan=2 | [[Helium-4|<sup>4</sup>He]]
! colspan=2 | [[Helium-3|<sup>3</sup>He]]
|-
! ''T''
! ''p''
! ''T''
! ''p''
|-
| [[Critical point]]
| 5.2 K
| 
| 3.3 K
| 
|-
| [[Boiling point]]
| 4.2 K
| 1 atm
| 3.2 K
| 1 atm
|-
| [[Melting point]] at<br/>minimum freezing pressure
| 1.0 K
| 25 bar
| 0.3 K
| 29 bar
|-
| [[λ point]] at<br/>saturated vapour pressure
| 2.17K
| 
| 1 mK
| 
|-
|}
{{main|Helium I|helium II}}
Liquid helium-4 can be condensed at 4.22&nbsp;K to a liquid phase now known as [[helium I]]. If cooling is continued (by allowing some of the liquid to evaporate), the physical properties of the liquid change drastically at about 2.2&nbsp;K (the exact value depends on the pressure). For a start, the visible bubbling of a boiling liquid disappears, although [[evaporation]] continues: the [[heat capacity]] increases ten-fold, the [[thermal conductivity]] increases a ''million-fold'' and the [[viscosity]] drops to virtually zero.

The low-temperature liquid phase is known as [[helium II]], and is a [[superfluid]] similar in nature to a [[Bose–Einstein condensate]], first discovered in 1938. The transition between the two liquid phases is a [[second-order phase transition]], and the transition temperature is known as the [[λ point]] after the shape of the plot of heat capacity vs. temperature, which resembles a Greek letter lambda. Perhaps the best known property of helium&nbsp;II is its propensity to form Rollin films, layers approximately 30&nbsp;nm thick that will climb up surfaces cool enough to support helium&nbsp;II even against the force of gravity: as such, the confinement of liquid helium below the λ&nbsp;point is particularly difficult.

[[Helium-3]] also forms a superfluid phase, discovered in 1972, but only at temperatures of one millikelvin or less. The difference in behaviour between the two isotopes is due to their nuclear spin: helium-4 atoms are [[boson]]s, with zero [[spin]], while helium-3 atoms are spin-½ [[fermion]]s, and the two types of particle obey different exclusion principles. For similar reasons, liquid helium-4 and liquid helium-3 are no longer miscible below about 0.9&nbsp;K.

Neither isotope can be solidified at pressures of 1&nbsp;atm. By raising the pressure to 25&nbsp;atm or higher, helium can be can be solidified at 1–1.5&nbsp;K. The solid form is unusually compressible, with a bulk modulus of around 50&nbsp;MPa (fifty-times more compressible than liquid water). Liquid and solid phases of helium have extremely low [[refractive index]]es, which makes it difficult to distinguish phase changes by visual means.

==Chemical properties==
Helium has the highest first [[ionization energy]] of any element ({{nowrap|2372.3&nbsp;kJ mol<sup>−1</sup>}}) and has almost unprecedented chemical inertness. No [[chemical compound]]s of helium are known in [[condensed phase]]s, although the [HeH]<sup>+</sup> cation (the protonation product of a helium atom) was first observed in 1925,<ref>{{citation | first1 = T. R. | last1 = Hogness | first2 = E. G. | last2 = Lunn | title = The Ionization of Hydrogen by Electron Impact as Interpreted by Positive Ray Analysis | journal = Phys. Rev. | year = 1925 | volume = 26 | pages = 44–55 | doi = 10.1103/PhysRev.26.44}}.</ref> and its neutral analogue is also known in the gas phase.

The low solubility of helium in water ({{nowrap|8.61&nbsp;cm<sup>3</sup> kg<sup>−1</sup>}} at 101.325&nbsp;kPa, 20&nbsp;°C)<ref name="G&E"/><ref>{{citation | title = Solubility of helium and neon in water and seawater | first = Ray F. | last = Weiss | journal = J. Chem. Eng. Data | year = 1971 | volume = 16 | issue = 2 | pages = 235–41 | doi = 10.1021/je60049a019}}.</ref> is important in its use as a component of [[breathing mixture]]s: helium, unlike [[nitrogen]] at high pressure, will not dissolve in [[blood serum]], and so poses fewer problems of degassing of the blood stream during decompression.

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

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

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