Editing Helium (section)

==Characteristics==

===Atom===
{{Main|Helium atom}}
[[File:Helium atom QM.svg|thumb|right|alt=Picture of a diffuse gray sphere with grayscale density decreasing from the center. Length scale about 1 Angstrom. An inset outlines the structure of the core, with two red and two blue atoms at the length scale of 1 femtometer.|'''The helium atom.''' Depicted are the [[atomic nucleus|nucleus]] (pink) and the [[electron cloud]] distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case.]]

====In quantum mechanics====
In the perspective of [[quantum mechanics]], helium is the second simplest [[atom]] to model, following the [[hydrogen atom]]. Helium is composed of two electrons in [[atomic orbital]]s surrounding a nucleus containing two protons and (usually) two neutrons. As in Newtonian mechanics, no system that consists of more than two particles can be solved with an exact analytical mathematical approach (see [[3-body problem]]) and helium is no exception. Thus, numerical mathematical methods are required, even to solve the system of one nucleus and two electrons. Such [[computational chemistry]] methods have been used to create a quantum mechanical picture of helium electron binding which is accurate to within < 2% of the correct value, in a few computational steps.<ref>{{Cite news|url=http://www.sjsu.edu/faculty/watkins/helium.htm|last=Watkins|first=Thayer|publisher=San Jose State University|title=The Old Quantum Physics of Niels Bohr and the Spectrum of Helium: A Modified Version of the Bohr Model|access-date=2009-06-24|archive-url=https://web.archive.org/web/20090526074018/http://www.sjsu.edu/faculty/watkins/helium.htm|archive-date=2009-05-26|url-status=live}}</ref> Such models show that each electron in helium partly screens the nucleus from the other, so that the [[effective nuclear charge]] ''Z''<sub>eff</sub> which each electron sees is about 1.69 units, not the 2 charges of a classic "bare" helium nucleus.

====Related stability of the helium-4 nucleus and electron shell====
The nucleus of the helium-4 atom is identical with an [[alpha particle]]. High-energy electron-scattering experiments show its charge to decrease exponentially from a maximum at a central point, exactly as does the charge density of helium's own [[electron cloud]]. This symmetry reflects similar underlying physics: the pair of neutrons and the pair of protons in helium's nucleus obey the same quantum mechanical rules as do helium's pair of electrons (although the nuclear particles are subject to a different nuclear binding potential), so that all these [[fermion]]s fully occupy 1s orbitals in pairs, none of them possessing orbital angular momentum, and each cancelling the other's intrinsic spin. This arrangement is thus energetically extremely stable for all these particles and has [[nuclear astrophysics|astrophysical]] implications.<ref name=Parker2022>{{cite journal |first1=M. C. |last1=Parker |first2=C. |last2=Jeynes |first3=W. N. |last3=Catford |title=Halo Properties in Helium Nuclei from the Perspective of Geometrical Thermodynamics |journal=Annalen der Physik |date=2022 |volume=534 |number=2100278 |doi=10.1002/andp.202100278 |doi-access=free|bibcode=2022AnP...53400278P }}</ref> Namely, adding another particle – proton, neutron, or alpha particle – would consume rather than release energy; all systems with [[mass number]] 5, as well as [[beryllium-8]] (comprising two alpha particles), are unbound.<ref name=8gap>{{cite journal |last1=Coc |first1=A. |last2=Vangioni |first2=E. |title=The triple-alpha reaction and the ''A''&nbsp;=&nbsp;8 gap in BBN and Population III stars |journal=Memorie della Società Astronomica Italiana |volume=85 |pages=124–129 |date=2014 |bibcode=2014MmSAI..85..124C |url=http://inspirehep.net/record/1338211/files/2014MmSAI..85..124C.pdf?version=1}}</ref>

For example, the stability and low energy of the electron cloud state in helium accounts for the element's chemical inertness, and also the lack of interaction of helium atoms with each other, producing the lowest melting and boiling points of all the elements. In a similar way, the particular energetic stability of the helium-4 nucleus, produced by similar effects, accounts for the ease of helium-4 production in atomic reactions that involve either heavy-particle emission or fusion. Some stable helium-3 (two protons and one neutron) is produced in fusion reactions from hydrogen, though its estimated abundance in the universe is about {{val|e=-5}} relative to helium-4.<ref name=Pitrou18/>

[[File:Binding energy curve - common isotopes.svg|thumb|right|Binding energy per nucleon of common isotopes. The binding energy per particle of helium-4 is significantly larger than all nearby nuclides.]]
The unusual stability of the helium-4 nucleus is also important [[Cosmology|cosmologically]]: it explains the fact that in the first few minutes after the [[Big Bang]], as the "soup" of free protons and neutrons which had initially been created in about 6:1 ratio cooled to the point that nuclear binding was possible, almost all first compound atomic nuclei to form were helium-4 nuclei. Owing to the relatively tight binding of helium-4 nuclei, its production consumed nearly all of the free neutrons in a few minutes, before they could beta-decay, and thus few neutrons were available to form heavier atoms such as lithium, beryllium, or boron. Helium-4 nuclear binding per nucleon is stronger than in any of these elements (see [[nucleogenesis]] and [[binding energy]]) and thus, once helium had been formed, no energetic drive was available to make elements 3, 4 and 5.<ref name=bbn99>{{cite journal |title=Cosmic lithium-beryllium-boron story |date=1999 |first1=E. |last1=Vangioni-Flam |first2=M. |last2=Cassé |doi=10.1023/A:1002197712862 |journal=Astrophysics and Space Science |volume=265 |pages=77–86 |arxiv=astro-ph/9902073|bibcode=1999Ap&SS.265...77V |s2cid=10627727 }}</ref> It is barely energetically favorable for helium to fuse into the next element with a lower energy per [[nucleon]], carbon. However, due to the short lifetime of the intermediate beryllium-8, this process requires three helium nuclei striking each other nearly simultaneously (see [[triple-alpha process]]).<ref name=8gap/> There was thus no time for significant carbon to be formed in the few minutes after the Big Bang, before the early expanding universe cooled to the temperature and pressure point where helium fusion to carbon was no longer possible. This left the early universe with a very similar ratio of hydrogen/helium as is observed today (3 parts hydrogen to 1 part helium-4 by mass), with nearly all the neutrons in the universe trapped in helium-4.

All heavier elements (including those necessary for rocky planets like the Earth, and for carbon-based or other life) have thus been created since the Big Bang in stars which were hot enough to fuse helium itself. All elements other than hydrogen and helium today account for only 2% of the mass of atomic matter in the universe. Helium-4, by contrast, comprises about 24% of the mass of the universe's ordinary matter—nearly all the ordinary matter that is not hydrogen.<ref name=Pitrou18>{{cite journal |title=Precision big bang nucleosynthesis with improved Helium-4 predictions |journal=Physics Reports |volume=754 |date=2018 |pages=1–66 |first1=C. |last1=Pitrou |first2=A. |last2=Coc |first3=J.-P. |last3=Uzan |first4=E. |last4=Vangioni |doi=10.1016/j.physrep.2018.04.005 |doi-access=free|arxiv=1801.08023 |bibcode=2018PhR...754....1P }}</ref><ref name=Hsyu20>{{cite journal |title=The PHLEK Survey: A New Determination of the Primordial Helium Abundance |first1=T. |last1=Hsyu |first2=R. J. |last2=Cooke |first3=J. X. |last3=Prochaska |first4=M. |last4=Bolte |date=2020 |journal=The Astrophysical Journal |volume=896 |number=77 |page=77 |doi=10.3847/1538-4357/ab91af |doi-access=free|arxiv=2005.12290 |bibcode=2020ApJ...896...77H }}</ref>

===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>

===Liquid phase===
{{Main|Liquid helium}}
[[File:Phase diagram of Helium-4-en.svg|thumb|upright=1.25|Phase diagram of helium-4. (Atmospheric pressure is about 0.1 MPa)]]
[[File:2 Helium.png|thumb|Liquefied helium. This helium is not only liquid, but has been cooled to the point of [[superfluid]]ity. The drop of liquid at the bottom of the glass represents helium spontaneously escaping from the container over the side, to empty out of the container. The energy to drive this process is supplied by the potential energy of the falling helium.]]

Helium liquifies when cooled below 4.2&nbsp;K at atmospheric pressure. Unlike any other element, however, helium remains liquid down to a temperature of [[absolute zero]]. This is a direct effect of quantum mechanics: specifically, the [[zero point energy]] of the system is too high to allow freezing. Pressures above about 25 atmospheres are required to freeze it. There are two liquid phases: Helium I is a conventional liquid, and Helium II, which occurs at a lower temperature, is a [[superfluid]].

====Helium I====
Below its [[boiling point]] of {{convert|4.22|K|C F}} and above the [[lambda point]] of {{convert|2.1768|K|C F}}, the [[isotope]] helium-4 exists in a normal colorless liquid state, called ''helium&nbsp;I''.<ref name="enc" /> Like other [[cryogenic]] liquids, helium&nbsp;I boils when it is heated and contracts when its temperature is lowered. Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered further. <!-- clarifyme / The rate of expansion decreases below the lambda point until about 1&nbsp;K is reached; at which point expansion completely stops and helium&nbsp;I starts to contract again. / if it is below the lambda point, should not it be helium II?-->

Helium&nbsp;I has a gas-like [[index of refraction]] of 1.026 which makes its surface so hard to see that floats of [[Expanded polystyrene|Styrofoam]] are often used to show where the surface is.<ref name="enc" /> This colorless liquid has a very low [[viscosity]] and a density of 0.145–0.125 g/mL (between about 0 and 4 K),<ref name="crc6120">{{RubberBible86th|page=6-120}}</ref> which is only one-fourth the value expected from [[classical physics]].<ref name="enc" /> [[Quantum mechanics]] is needed to explain this property and thus both states of liquid helium (helium I and helium II) are called ''quantum fluids'', meaning they display atomic properties on a macroscopic scale. This may be an effect of its boiling point being so close to absolute zero, preventing random molecular motion ([[thermal energy]]) from masking the atomic properties.<ref name="enc" />

====Helium II====
{{main|Superfluid helium-4}}
Liquid helium below its lambda point (called ''helium&nbsp;II'') exhibits very unusual characteristics. Due to its high [[thermal conductivity]], when it boils, it does not bubble but rather evaporates directly from its surface. [[Helium-3]] also has a [[superfluid]] phase, but only at much lower temperatures; as a result, less is known about the properties of the isotope.<ref name="enc" />
[[File:helium-II-creep.svg|thumb|upright|Unlike ordinary liquids, helium&nbsp;II will creep along surfaces in order to reach an equal level; after a short while, the levels in the two containers will equalize. The [[Rollin film]] also covers the interior of the larger container; if it were not sealed, the helium&nbsp;II would creep out and escape.<ref name="enc" />|alt=A cross-sectional drawing showing one vessel inside another. There is a liquid in the outer vessel, and it tends to flow into the inner vessel over its walls.]]

Helium&nbsp;II is a superfluid, a [[Macroscopic quantum phenomena|quantum mechanical state]] of matter with strange properties. For example, when it flows through capillaries as thin as 10 to 100 [[Nanometre|nm]] it has no measurable [[viscosity]].<ref name="nbb" /> However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Existing theory explains this using the ''two-fluid model'' for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a [[ground state]], which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.<ref>{{Cite journal|doi = 10.1006/aphy.2000.6019 |title = Microscopic Theory of Superfluid Helium |journal = Annals of Physics |volume = 281 |issue = 1–2 |date = 2000|pages = 636–705 12091211 |author = Hohenberg, P. C. |author2 = Martin, P. C.|bibcode = 2000AnPhy.281..636H }}</ref>

In the ''fountain effect'', a chamber is constructed which is connected to a reservoir of helium&nbsp;II by a [[sintering|sintered]] disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.<ref>{{cite web|last=Warner|first=Brent|url=http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html |title=Introduction to Liquid Helium |publisher=NASA|access-date=2007-01-05 |url-status=dead|archive-url=https://web.archive.org/web/20050901062951/http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html |archive-date=2005-09-01}}</ref>

The thermal conductivity of helium&nbsp;II is greater than that of any other known substance, a million times that of helium&nbsp;I and several hundred times that of [[copper]].<ref name="enc" /> This is because heat conduction occurs by an exceptional quantum mechanism. Most materials that conduct heat well have a [[valence band]] of free electrons which serve to transfer the heat. Helium&nbsp;II has no such valence band but nevertheless conducts heat well. The [[heat transfer|flow of heat]] is governed by equations that are similar to the [[wave equation]] used to characterize sound propagation in air. When heat is introduced, it moves at 20 meters per second at 1.8&nbsp;K through helium&nbsp;II as waves in a phenomenon known as ''[[second sound]]''.<ref name="enc" />

Helium&nbsp;II also exhibits a creeping effect. When a surface extends past the level of helium&nbsp;II, the helium&nbsp;II moves along the surface, against the force of [[gravity]]. Helium&nbsp;II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30&nbsp;nm-thick film regardless of surface material. This film is called a [[Rollin film]] and is named after the man who first characterized this trait, [[Bernard V. Rollin]].<ref name="enc" /><ref>{{Cite journal|doi = 10.1103/PhysRev.76.1209 |title = Rollin Film Rates in Liquid Helium |journal = Physical Review |volume = 76 |issue = 8 |pages = 1209–1211|date = 1949 |author = Fairbank, H. A. |author2 = Lane, C. T. |bibcode=1949PhRv...76.1209F}}</ref><ref>{{Cite journal|doi = 10.1016/S0031-8914(39)80013-1 |title = On the 'film' phenomenon of liquid helium II |journal = Physica |volume = 6 |issue = 2 |date = 1939 |pages = 219–230 |author = Rollin, B. V. |author2 = Simon, F. |bibcode=1939Phy.....6..219R}}</ref> As a result of this creeping behavior and helium&nbsp;II's ability to leak rapidly through tiny openings, it is very difficult to confine. Unless the container is carefully constructed, the helium&nbsp;II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate. Waves propagating across a Rollin film are governed by the same equation as [[gravity wave]]s in shallow water, but rather than gravity, the restoring force is the [[van der Waals force]].<ref>{{cite web |author = Ellis, Fred M. |url = http://fellis.web.wesleyan.edu/research/thrdsnd.html |title = Third sound |publisher = Wesleyan Quantum Fluids Laboratory |date = 2005 |access-date = 2008-07-23 |archive-url = https://web.archive.org/web/20070621202145/http://fellis.web.wesleyan.edu/research/thrdsnd.html |archive-date = 2007-06-21 |url-status = live }}</ref> These waves are known as ''[[third sound]]''.<ref>{{Cite journal|doi = 10.1103/PhysRev.188.370 |title = Hydrodynamics and Third Sound in Thin He II Films |journal = Physical Review |volume = 188 |issue = 1|date = 1949 |pages = 370–384|author = Bergman, D.|bibcode = 1969PhRv..188..370B }}</ref><!-- "van", see cite itself and [[Talk:Van der Waals#Van should be capitalized unless preceded by first name]] rebuttal -->

===Solid phases===
Helium remains liquid down to [[absolute zero]] at atmospheric pressure, but it freezes at high pressure. Solid helium requires a temperature of 1–1.5&nbsp;K (about −272&nbsp;°C or −457&nbsp;°F) at about 25 bar (2.5&nbsp;MPa) of pressure.<ref>{{cite web|date = 2005-10-05 |url = http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm |title = Solid Helium |publisher = Department of Physics [[University of Alberta]]|access-date=2008-07-20| archive-url = https://web.archive.org/web/20080531145546/http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm| archive-date = May 31, 2008}}</ref> It is often hard to distinguish solid from liquid helium since the [[refractive index]] of the two phases are nearly the same. The solid has a sharp [[melting point]] and has a [[crystal]]line structure, but it is highly [[Compressibility|compressible]]; applying pressure in a laboratory can decrease its volume by more than 30%.<ref name="LANL.gov">{{RubberBible86th}}</ref> With a [[bulk modulus]] of about 27 [[megapascal|MPa]]<ref>{{Cite journal|author = Grilly, E. R.|title = Pressure-volume-temperature relations in liquid and solid 4He |journal = Journal of Low Temperature Physics|volume = 11 |issue = 1–2 |pages = 33–52 |doi = 10.1007/BF00655035|date = 1973|bibcode = 1973JLTP...11...33G|s2cid = 189850188 }}</ref> it is ~100 times more compressible than water. Solid helium has a density of {{val|0.214|0.006|u=g/cm<sup>3</sup>}} at 1.15&nbsp;K and 66&nbsp;atm; the projected density at 0&nbsp;K and 25 bar (2.5 MPa) is {{val|0.187|0.009|u=g/cm<sup>3</sup>}}.<ref>{{Cite journal|author = Henshaw, D. B. |title = Structure of Solid Helium by Neutron Diffraction |journal = Physical Review Letters |volume = 109 |issue = 2 |pages = 328–330 |doi = 10.1103/PhysRev.109.328 |date = 1958|bibcode = 1958PhRv..109..328H }}</ref> At higher temperatures, helium will solidify with sufficient pressure. At room temperature, this requires about 114,000 atm.<ref name="Pinceaux1979">{{cite journal |last1=Pinceaux |first1=J.-P. |last2=Maury |first2=J.-P. |last3=Besson |first3=J.-M. |title=Solidification of helium, at room temperature under high pressure |journal=Journal de Physique Lettres |date=1979 |volume=40 |issue=13 |pages=307–308 |doi=10.1051/jphyslet:019790040013030700|s2cid=40164915 |url=https://hal.archives-ouvertes.fr/jpa-00231630/file/ajp-jphyslet_1979_40_13_307_0.pdf }}</ref>

Helium-4 and helium-3 both form several crystalline solid phases, all requiring at least 25&nbsp;bar. They both form an α phase, which has a [[Hexagonal crystal family#Hexagonal close packed|hexagonal close-packed]] (hcp) crystal structure, a β phase, which is [[Cubic crystal system|face-centered cubic]] (fcc), and a γ phase, which is [[Cubic crystal system|body-centered cubic]] (bcc).<ref name="Keller 1969">{{cite book | last=Keller | first=William E. | chapter=Compressed He3 and He4 | title=Helium-3 and Helium-4 | publisher=Springer US | publication-place=Boston, MA | year=1969 | isbn=978-1-4899-6232-4 | doi=10.1007/978-1-4899-6485-4_9 | pages=347–404}}</ref>

===Isotopes===
{{Main|Isotopes of helium}}
There are nine known [[isotope]]s of helium of which two, [[helium-3]] and [[helium-4]], are [[stable isotope|stable]]. In the Earth's atmosphere, one atom is {{chem|3|He}} for every million that are {{chem|4|He}}.<ref name="nbb">{{Cite book| author = Emsley, John| title = Nature's Building Blocks| publisher = Oxford University Press| date = 2001| location = Oxford| pages = 175–179| isbn = 978-0-19-850341-5}}</ref> Unlike most elements, helium's isotopic abundance varies greatly by origin, due to the different formation processes. The most common isotope, helium-4, is produced on Earth by [[alpha decay]] of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its [[nucleon]]s are arranged into [[Nuclear shell model|complete shells]]. It was also formed in enormous quantities during [[Big Bang nucleosynthesis]].<ref name="bigbang" />

Helium-3 is present on Earth only in trace amounts. Most of it has been present since Earth's formation, though some falls to Earth trapped in [[cosmic dust]].<ref name="heliumfundamentals">{{cite web |url = http://www.mantleplumes.org/HeliumFundamentals.html |title = Helium Fundamentals |author = Anderson, Don L. |author2 = Foulger, G. R. |author3 = Meibom, A. |date = 2006-09-02 |access-date = 2008-07-20 |publisher = MantlePlumes.org |archive-url = https://web.archive.org/web/20070208194933/http://www.mantleplumes.org/HeliumFundamentals.html |archive-date = 2007-02-08 |url-status = live }}</ref> Trace amounts are also produced by the [[beta decay]] of [[tritium]].<ref>{{Cite journal|title= Half-Life of Tritium| journal=Physical Review|volume= 72|issue= 10|date= 1947| pages= 972|last= Novick|first=Aaron| doi=10.1103/PhysRev.72.972.2|bibcode = 1947PhRv...72..972N }}</ref> Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten, and these ratios can be used to investigate the origin of rocks and the composition of the Earth's [[Mantle (geology)|mantle]].<ref name="heliumfundamentals" /> {{chem|3|He}} is much more abundant in stars as a product of nuclear fusion. Thus in the [[interstellar medium]], the proportion of {{chem|3|He}} to {{chem|4|He}} is about 100 times higher than on Earth.<ref>{{Cite journal|title=Isotopic Composition and Abundance of Interstellar Neutral Helium Based on Direct Measurements| journal=Astrophysics| volume=45| issue=2|date=2002| pages=131–142| last1=Zastenker | first1=G. N. | doi=10.1023/A:1016057812964|bibcode = 2002Ap.....45..131Z|last2=Salerno | first2=E. | last3=Buehler |first3=F.|last4=Bochsler | first4=P.|last5=Bassi | first5=M. |last6=Agafonov | first6=Yu. N. |last7=Eisomont| first7=N. A. |last8=Khrapchenkov | first8=V. V. | last9=Busemann | first9=H.| s2cid=116957905| display-authors = 8 }}</ref> Extraplanetary material, such as [[Moon|lunar]] and [[asteroid]] [[regolith]], have trace amounts of helium-3 from being bombarded by [[solar wind]]s. The [[Moon]]'s surface contains helium-3 at concentrations on the order of 10 [[Parts per billion|ppb]], much higher than the approximately 5 [[Parts per trillion|ppt]] found in the Earth's atmosphere.<ref>{{cite web|url = http://fti.neep.wisc.edu/research/he3|title = Lunar Mining of Helium-3|date = 2007-10-19|access-date = 2008-07-09|publisher = Fusion Technology Institute of the University of Wisconsin-Madison|archive-url = https://web.archive.org/web/20100609234057/http://fti.neep.wisc.edu/research/he3|archive-date = 2010-06-09|url-status = live}}</ref><ref>{{cite journal|url= http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf|title= The estimation of helium-3 probable reserves in lunar regolith|author= Slyuta, E. N.|author2= Abdrakhimov, A. M.|author3= Galimov, E. M.|journal= Lunar and Planetary Science Conference|issue= 1338|pages= 2175|date= 2007|access-date= 2008-07-20|bibcode= 2007LPI....38.2175S|archive-url= https://web.archive.org/web/20080705122316/http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2175.pdf|archive-date= 2008-07-05|url-status= live}}</ref> A number of people, starting with Gerald Kulcinski in 1986,<ref>{{Cite news|url = http://www.thespacereview.com/article/536/1|title = A fascinating hour with Gerald Kulcinski|author = Hedman, Eric R.|date = 2006-01-16|work = The Space Review|access-date = 2008-07-20|archive-url = https://web.archive.org/web/20110109082500/http://thespacereview.com/article/536/1|archive-date = 2011-01-09|url-status = dead}}</ref> have proposed to explore the Moon, mine lunar regolith, and use the helium-3 for [[Nuclear fusion|fusion]].

Liquid helium-4 can be cooled to about {{convert|1|K|C F}} using [[evaporative cooling]] in a [[1-K pot]]. Similar cooling of helium-3, which has a lower boiling point, can achieve about {{val|0.2|u=kelvin}} in a [[Helium-3#Cryogenics|helium-3 refrigerator]]. Equal mixtures of liquid {{chem|3|He}} and {{chem|4|He}} below {{val|0.8|u=K}} separate into two immiscible phases due to their dissimilarity (they follow different [[quantum statistics]]: helium-4 atoms are [[boson]]s while helium-3 atoms are [[fermion]]s).<ref name = enc/> [[Dilution refrigerator]]s use this immiscibility to achieve temperatures of a few millikelvins.<ref>{{Cite journal | doi = 10.1016/j.cryogenics.2021.103390|issn=0011-2275| title = Development of Dilution refrigerators – A review | journal = Cryogenics| volume = 121| year = 2022| last1 = Zu | first1 = H.| last2 = Dai | first2 = W.| last3 = de Waele | first3 = A.T.A.M.|s2cid=244005391}}</ref>

It is possible to produce [[exotic helium isotopes]], which rapidly decay into other substances. The shortest-lived heavy helium isotope is the [[nuclear drip line|unbound]] helium-10 with a [[half-life]] of {{val|2.6|(4)|e=-22|u=s}}.{{NUBASE2020|ref}} Helium-6 decays by emitting a [[beta particle]] and has a half-life of 0.8 second. Helium-7 and helium-8 are created in certain [[nuclear reaction]]s.<ref name="enc" /> Helium-6 and helium-8 are known to exhibit a [[nuclear halo]].<ref name = enc/>

=== Properties ===
Table of thermal and physical properties of helium gas at atmospheric pressure:<ref>{{Cite book |last=Holman |first=Jack P. |title=Heat Transfer |publisher=McGraw-Hill Companies, Inc. |year=2002 |isbn=9780072406559 |edition=9th |location=New York, NY |pages=600–606 |language=English}}</ref><ref>{{Cite book |last1=Incropera |last2=Dewitt |last3=Bergman |last4=Lavigne |first1=Frank P. |first2=David P. |first3=Theodore L. |first4=Adrienne S. |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 }}</ref>
{|class="wikitable mw-collapsible mw-collapsed"
|[[Temperature]] (K)
|[[Density]] (kg/m^3)
|[[Specific heat]] (kJ/kg °C)
|[[Dynamic viscosity]] (kg/m s)
|[[Kinematic viscosity]] (m^2/s)
|[[Thermal conductivity]] (W/m °C)
|[[Thermal diffusivity]] (m^2/s)
|[[Prandtl number]]
|-
|100
| 
|5.193
|9.63E-06
|1.98E-05
|0.073
|2.89E-05
|0.686
|-
|120
|0.406
|5.193
|1.07E-05
|2.64E-05
|0.0819
|3.88E-05
|0.679
|-
|144
|0.3379
|5.193
|1.26E-05
|3.71E-05
|0.0928
|5.28E-05
|0.7
|-
|200
|0.2435
|5.193
|1.57E-05
|6.44E-05
|0.1177
|9.29E-05
|0.69
|-
|255
|0.1906
|5.193
|1.82E-05
|9.55E-05
|0.1357
|1.37E-04
|0.7
|-
|366
|0.1328
|5.193
|2.31E-05
|1.74E-04
|0.1691
|2.45E-04
|0.71
|-
|477
|0.10204
|5.193
|2.75E-05
|2.69E-04
|0.197
|3.72E-04
|0.72
|-
|589
|0.08282
|5.193
|3.11E-05
|3.76E-04
|0.225
|5.22E-04
|0.72
|-
|700
|0.07032
|5.193
|3.48E-05
|4.94E-04
|0.251
|6.66E-04
|0.72
|-
|800
|0.06023
|5.193
|3.82E-05
|6.34E-04
|0.275
|8.77E-04
|0.72
|-
|900
|0.05451
|5.193
|4.14E-05
|7.59E-04
|0.33
|1.14E-03
|0.687
|-
|1000
| 
|5.193
|4.46E-05
|9.14E-04
|0.354
|1.40E-03
|0.654
|}