Editing Helium (section)

==Compounds==
{{main|Helium compounds}}
[[File:Helium-hydride-cation-3D-SF.png|thumb|Structure of the [[helium hydride ion]], HHe<sup>+</sup>]]
[[File:Fluoroheliate-ion-3D-vdW.png|thumb|Structure of the suspected fluoroheliate anion, OHeF<sup>−</sup>]]
Helium has a [[Valence (chemistry)|valence]] of zero and is chemically unreactive under all normal conditions.<ref name="LANL.gov" /> It is an electrical insulator unless [[ion]]ized. As with the other noble gases, helium has metastable [[energy level]]s that allow it to remain ionized in an electrical discharge with a [[voltage]] below its [[ionization potential]].<ref name="enc" /> Helium can form unstable [[compound (chemistry)|compounds]], known as [[excimer]]s, with tungsten, iodine, fluorine, sulfur, and phosphorus when it is subjected to a [[glow discharge]], to electron bombardment, or reduced to [[Plasma physics|plasma]] by other means. The molecular compounds HeNe, HgHe<sub>10</sub>, and WHe<sub>2</sub>, and the molecular ions {{chem|He|2|+}}, {{chem|He|2|2+}}, {{chem|link=Helium hydride ion|HeH|+}}, and {{chem|HeD|+}} have been created this way.<ref>{{Cite journal|title = Massenspektrographische Untersuchungen an Wasserstoff- und Heliumkanalstrahlen ({{chem|H|3|+}}, {{chem|H|2|-}}, {{chem|HeH|+}}, {{chem|HeD|+}}, {{chem|He|-}}) |author = Hiby, Julius W. |journal = [[Annalen der Physik]] |volume = 426 |issue = 5 |pages = 473–487 |date = 1939 |doi = 10.1002/andp.19394260506 |bibcode = 1939AnP...426..473H }}</ref> HeH<sup>+</sup> is also stable in its ground state but is extremely reactive—it is the strongest [[Brønsted–Lowry acid–base theory|Brønsted acid]] known, and therefore can exist only in isolation, as it will protonate any molecule or counteranion it contacts. This technique has also produced the neutral molecule He<sub>2</sub>, which has a large number of [[spectral band|band systems]], and HgHe, which is apparently held together only by polarization forces.<ref name="enc" />

[[Van der Waals compound]]s of helium can also be formed with cryogenic helium gas and atoms of some other substance, such as [[LiHe]] and [[dihelium|He<sub>2</sub>]].<ref name="fr13">{{cite magazine|last1=Friedrich|first1=Bretislav|title=A Fragile Union Between Li and He Atoms|magazine=Physics|date=8 April 2013|volume=6|page=42|doi=10.1103/Physics.6.42|bibcode=2013PhyOJ...6...42F|url=https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.110.153201|access-date=24 August 2019|archive-url=https://web.archive.org/web/20170829154727/https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.110.153201|archive-date=29 August 2017|url-status=live|hdl=11858/00-001M-0000-000E-F3CF-5|hdl-access=free}}</ref>

Theoretically, other true compounds may be possible, such as helium fluorohydride (HHeF), which would be analogous to [[Argon fluorohydride|HArF]], discovered in 2000.<ref>{{Cite journal|title = Prediction of a Metastable Helium Compound: HHeF |author = Wong, Ming Wah|journal = [[Journal of the American Chemical Society]] |volume = 122 |issue = 26 |pages = 6289–6290 |date = 2000 |doi = 10.1021/ja9938175| bibcode=2000JAChS.122.6289W }}</ref> Calculations show that two new compounds containing a helium-oxygen bond could be stable.<ref>{{Cite journal|title = On Chemical Bonding Between Helium and Oxygen|first = W.|last = Grochala|journal = Polish Journal of Chemistry|volume = 83|pages = 87–122|date =2009}}</ref> Two new molecular species, predicted using theory, CsFHeO and N(CH<sub>3</sub>)<sub>4</sub>FHeO, are derivatives of a metastable FHeO<sup>−</sup> anion first theorized in 2005 by a group from Taiwan.<ref>{{cite web|url = http://www.uw.edu.pl/en/strony/news/chemist.pdf|archive-url = https://web.archive.org/web/20090319180147/http://www.uw.edu.pl/en/strony/news/chemist.pdf|archive-date = 2009-03-19|title = Collapse of helium's chemical nobility predicted by Polish chemist|access-date = 2009-05-15}}</ref>

Helium atoms have been inserted into the hollow carbon cage molecules (the [[fullerene]]s) by heating under high pressure. The [[endohedral fullerene|endohedral fullerene molecules]] formed are stable at high temperatures. When chemical derivatives of these fullerenes are formed, the helium stays inside.<ref>{{Cite journal|title = Stable Compounds of Helium and Neon: He@C<sub>60</sub> and Ne@C<sub>60</sub> |author = Saunders, Martin |author2 = Jiménez-Vázquez, Hugo A. |author3 = Cross, R. James |author4 = Poreda, Robert J. |journal = Science |volume = 259 |issue = 5100 |pages = 1428–1430 |date = 1993 |doi = 10.1126/science.259.5100.1428 |pmid = 17801275|bibcode = 1993Sci...259.1428S |s2cid = 41794612 }}</ref> If [[helium-3]] is used, it can be readily observed by helium [[nuclear magnetic resonance spectroscopy]].<ref>{{Cite journal|title = Probing the interior of fullerenes by <sup>3</sup>He NMR spectroscopy of endohedral <sup>3</sup>He@C<sub>60</sub> and <sup>3</sup>He@C<sub>70</sub> |last1=Saunders|first1=Martin|journal = Nature |volume = 367|issue = 6460|pages = 256–258 |date = 1994 |doi = 10.1038/367256a0|bibcode = 1994Natur.367..256S|first2 = Hugo A.|last2=Jiménez-Vázquez|first3 = R. James|last3=Cross|first4 = Stanley|last4=Mroczkowski|first5 = Darón I.|last5=Freedberg|first6 = Frank A. L.|last6=Anet|s2cid=4273677}}</ref> Many fullerenes containing helium-3 have been reported. Although the helium atoms are not attached by covalent or ionic bonds, these substances have distinct properties and a definite composition, like all stoichiometric chemical compounds.

Under high pressures helium can form compounds with various other elements. Helium-nitrogen [[clathrate]] (He(N<sub>2</sub>)<sub>11</sub>) crystals have been grown at room temperature at pressures ca. 10 GPa in a [[diamond anvil cell]].<ref>{{cite journal|doi=10.1038/358046a0|title=A high-pressure van der Waals compound in solid nitrogen-helium mixtures|journal=Nature|volume=358|issue=6381|pages=46–48|year=1992|last1=Vos|first1=W. L.|last2=Finger|first2=L. W.|last3=Hemley|first3=R. J.|last4=Hu|first4=J. Z.|last5=Mao|first5=H. K.|last6=Schouten|first6=J. A.|bibcode=1992Natur.358...46V|s2cid=4313676}}</ref> The [[Insulator (electricity)|insulating]] [[electride]] [[Disodium helide|Na<sub>2</sub>He]] has been shown to be thermodynamically stable at pressures above 113 GPa. It has a [[fluorite]] structure.<ref name="DongOganov2017">{{cite journal |last1=Dong|first1=Xiao |last2=Oganov|first2=Artem R. |last3=Goncharov|first3=Alexander F. |last4=Stavrou|first4=Elissaios |last5=Lobanov|first5=Sergey |last6=Saleh|first6=Gabriele |last7=Qian|first7=Guang-Rui |last8=Zhu|first8=Qiang |last9=Gatti|first9=Carlo |last10=Deringer|first10=Volker L. |last11=Dronskowski|first11=Richard |last12=Zhou|first12=Xiang-Feng |last13=Prakapenka|first13=Vitali B. |last14=Konôpková|first14=Zuzana |last15=Popov|first15=Ivan A. |last16=Boldyrev|first16=Alexander I. |last17=Wang|first17=Hui-Tian |title=A stable compound of helium and sodium at high pressure |journal=Nature Chemistry |volume=9 |issue=5 |pages=440–445 |year=2017 |issn=1755-4330 |doi=10.1038/nchem.2716 |pmid=28430195 |bibcode=2017NatCh...9..440D|arxiv=1309.3827 |s2cid=20459726 }}</ref>