Editing Helium (section)

===Gas and plasma phases===
[[File:HeTube.jpg|thumb|left|upright|Helium discharge tube shaped into 'He', the element's symbol.|alt=Illuminated light red gas discharge tubes shaped as letters H and e]]
Helium is the second least reactive noble gas after [[neon]], and thus the second least reactive of all elements.<ref>{{Cite book
|url=https://books.google.com/books?id=IoFzgBSSCwEC&pg=PA70|title=Modelling Marvels|last=Lewars|first=Errol G. |publisher=Springer|date=2008|isbn=978-1-4020-6972-7|pages=70–71|bibcode=2008moma.book.....L}}</ref> It is [[chemically inert]] and monatomic in all standard conditions. Because of helium's relatively low molar (atomic) mass, its [[thermal conductivity]], [[specific heat]], and [[Speed of sound|sound speed]] in the gas phase are all greater than any other gas except [[hydrogen]]. For these reasons and the small size of helium monatomic molecules, helium [[diffusion|diffuses]] through solids at a rate three times that of air and around 65% that of hydrogen.<ref name="enc" />

Helium is the least water-[[solubility|soluble]] monatomic gas,<ref>{{Cite journal|title = Solubility of helium and neon in water and seawater|last = Weiss|first=Ray F.| date = 1971| journal = J. Chem. Eng. Data|volume = 16|issue = 2|pages = 235–241 |doi = 10.1021/je60049a019}}</ref> and one of the least water-soluble of any gas ([[Tetrafluoromethane|CF<sub>4</sub>]], [[Sulfur hexafluoride|SF<sub>6</sub>]], and [[Octafluorocyclobutane|C<sub>4</sub>F<sub>8</sub>]] have lower mole fraction solubilities: 0.3802, 0.4394, and 0.2372 x<sub>2</sub>/10<sup>−5</sup>, respectively, versus helium's 0.70797 x<sub>2</sub>/10<sup>−5</sup>),<ref>{{Cite journal|title = Solubility of gases in water: Correlation between solubility and the number of water molecules in the first solvation shell |last1 = Scharlin|first1=P. |last2 = Battino|first2=R.|last3=Silla|first3=E. |last4 = Tuñón|first4=I. |last5 = Pascual-Ahuir|first5=J. L.| date = 1998| journal = Pure and Applied Chemistry |volume = 70|issue = 10|pages = 1895–1904 |doi= 10.1351/pac199870101895 |s2cid = 96604119|doi-access = free}}</ref> and helium's [[index of refraction]] is closer to unity than that of any other gas.<ref>{{Cite journal|title = Using helium as a standard of refractive index: correcting errors in a gas refractometer |last1 = Stone|first1=Jack A. |last2 = Stejskal|first2=Alois|date = 2004| journal = Metrologia|volume = 41|issue = 3|pages = 189–197 |doi =10.1088/0026-1394/41/3/012|bibcode = 2004Metro..41..189S | s2cid=250809634 |url=https://www.researchgate.net/publication/231064946}}</ref> Helium has a negative [[Joule–Thomson coefficient]] at normal ambient temperatures, meaning it heats up when allowed to freely expand. Only below its [[Joule–Thomson inversion temperature]] (of about 32 to 50&nbsp;K at 1 atmosphere) does it cool upon free expansion.<ref name="enc" /> Once precooled below this temperature, helium can be liquefied through expansion cooling.

Most extraterrestrial helium is [[plasma (physics)|plasma]] in stars, with properties quite different from those of atomic helium. In a plasma, helium's electrons are not bound to its nucleus, resulting in very high electrical conductivity, even when the gas is only partially ionized. The charged particles are highly influenced by magnetic and electric fields. For example, in the [[solar wind]] together with ionized hydrogen, the particles interact with the Earth's [[magnetosphere]], giving rise to [[Birkeland current]]s and the [[aurora]].<ref>{{Cite journal|title = Helium isotopes in an aurora|last1 = Buhler|first1=F.|last2 = Axford|first2=W. I.|last3 = Chivers|first3=H. J. A.|last4 = Martin|first4=K.| date = 1976|journal = J. Geophys. Res.|volume = 81|issue = 1|pages = 111–115|doi = 10.1029/JA081i001p00111|bibcode=1976JGR....81..111B}}</ref>