Editing Noble gas (section)

===Compounds===
{{Main|Noble gas compound}}
[[File:Xenon-tetrafluoride-3D-vdW.png|thumb|left|Structure of [[xenon tetrafluoride]] ({{chem|Xe||F|4}}), one of the first noble gas compounds to be discovered|alt=A model of planar chemical molecule with a blue center atom (Xe) symmetrically bonded to four peripheral atoms (fluorine).]]

The noble gases show extremely low [[reactivity (chemistry)|chemical reactivity]]; consequently, only a few hundred [[noble gas compound]]s have been formed. Neutral [[chemical compound|compounds]] in which helium and neon are involved in [[chemical bond]]s have not been formed (although some helium-containing [[ion]]s exist and there is some theoretical evidence for a few neutral helium-containing ones), while xenon, krypton, and argon have shown only minor reactivity.<ref name=Ngcomp>{{cite journal|last=Grochala|first=Wojciech|title=Atypical compounds of gases, which have been called noble|journal=[[Chemical Society Reviews]]|year=2007|issue=10|pages=1632–1655|doi=10.1039/b702109g|volume=36|pmid=17721587|url=http://www.chem.uw.edu.pl/people/WGrochala/Ng_essay.pdf?origin%3Dpublication_detail|access-date=25 October 2017|archive-date=26 October 2017|archive-url=https://web.archive.org/web/20171026111020/http://beta.chem.uw.edu.pl/people/WGrochala/Ng_essay.pdf?origin%3Dpublication_detail|url-status=dead}}</ref> The reactivity follows the order Ne < He < Ar < Kr < Xe < Rn ≪ Og.

In 1933, [[Linus Pauling]] predicted that the heavier noble gases could form compounds with fluorine and oxygen. He predicted the existence of [[krypton hexafluoride]] ({{chem|KrF|6}}) and [[xenon hexafluoride]] ({{chem|XeF|6}}) and speculated that [[xenon octafluoride]] ({{chem|XeF|8}}) might exist as an unstable compound, and suggested that [[xenic acid]] could form [[perxenate]] salts.<ref>{{cite journal|title=The Formulas of Antimonic Acid and the Antimonates|last=Pauling|first=Linus|journal=[[Journal of the American Chemical Society]]|volume=55|issue=5|pages=1895–1900|year=1933| doi=10.1021/ja01332a016|bibcode=1933JAChS..55.1895P }}</ref><ref name="Holloway">{{harvnb|Holloway|1968}}</ref> These predictions were shown to be generally accurate, except that {{chem|XeF|8}} is now thought to be both [[thermodynamic stability|thermodynamically]] and [[kinetic theory of gases|kinetically]] unstable.<ref>{{cite journal|last=Seppelt|first=Konrad|year=1979|title=Recent developments in the Chemistry of Some Electronegative Elements|journal=[[Accounts of Chemical Research]]|volume=12|pages=211–216|doi=10.1021/ar50138a004|issue=6}}</ref>

[[Xenon compounds]] are the most numerous of the noble gas compounds that have been formed.<ref>{{cite journal|last=Moody|first=G. J.|title=A Decade of Xenon Chemistry|journal=Journal of Chemical Education|year=1974|issue=10|volume=51|pages=628–630| url=http://www.eric.ed.gov/ERICWebPortal/custom/portlets/recordDetails/detailmini.jsp?_nfpb=true&_&ERICExtSearch_SearchValue_0=EJ111480&ERICExtSearch_SearchType_0=no&accno=EJ111480|access-date=16 October 2007|doi=10.1021/ed051p628|bibcode= 1974JChEd..51..628M }}</ref> Most of them have the xenon atom in the [[oxidation state]] of +2, +4, +6, or +8 bonded to highly [[electronegative]] atoms such as fluorine or oxygen, as in [[xenon difluoride]] ({{chem|XeF|2}}), [[xenon tetrafluoride]] ({{chem|XeF|4}}), [[xenon hexafluoride]] ({{chem|XeF|6}}), [[xenon tetroxide]] ({{chem|XeO|4}}), and [[sodium perxenate]] ({{chem|Na|4|XeO|6}}). Xenon reacts with fluorine to form numerous xenon fluorides according to the following equations:

::Xe + F<sub>2</sub> → XeF<sub>2</sub>
::Xe + 2F<sub>2</sub> → XeF<sub>4</sub>
::Xe + 3F<sub>2</sub> → XeF<sub>6</sub>

Some of these compounds have found use in [[chemical synthesis]] as [[oxidizing agent]]s; {{chem|XeF|2}}, in particular, is commercially available and can be used as a [[fluorination|fluorinating]] agent.<ref>{{cite journal |title=Fluorination with XeF<sub>2</sub>. 44. Effect of Geometry and Heteroatom on the Regioselectivity of Fluorine Introduction into an Aromatic Ring |author1=Zupan, Marko |author2=Iskra, Jernej |author3=Stavber, Stojan |journal=J. Org. Chem. |year=1998 |volume=63 |issue=3 |pages=878–880 |doi=10.1021/jo971496e |pmid=11672087}}</ref> As of 2007, about five hundred compounds of xenon bonded to other elements have been identified, including organoxenon compounds (containing xenon bonded to carbon), and xenon bonded to nitrogen, chlorine, gold, mercury, and xenon itself.<ref name=Ngcomp/><ref>{{harvnb|Harding|2002|pp=90–99}}</ref> Compounds of xenon bound to boron, hydrogen, bromine, iodine, beryllium, sulphur, titanium, copper, and silver have also been observed but only at low temperatures in noble gas [[matrix isolation|matrices]], or in supersonic noble gas jets.<ref name=Ngcomp/>

Radon is more reactive than xenon, and forms chemical bonds more easily than xenon does. However, due to the high radioactivity and short half-life of [[isotopes of radon|radon isotopes]], only a few [[fluoride]]s and [[oxide]]s of radon have been formed in practice.<ref>.{{cite journal|title=The Chemistry of Radon|volume=51|journal=Russian Chemical Reviews|year=1982|issue=1|pages=12–20|author1=Avrorin, V. V. |author2=Krasikova, R. N. |author3=Nefedov, V. D. |author4=Toropova, M. A. |doi=10.1070/RC1982v051n01ABEH002787|bibcode= 1982RuCRv..51...12A |s2cid=250906059 }}</ref> Radon goes further towards metallic behavior than xenon; the difluoride RnF<sub>2</sub> is highly ionic, and cationic Rn<sup>2+</sup> is formed in halogen fluoride solutions. For this reason, kinetic hindrance makes it difficult to oxidize radon beyond the +2 state. Only tracer experiments appear to have succeeded in doing so, probably forming RnF<sub>4</sub>, RnF<sub>6</sub>, and RnO<sub>3</sub>.<ref name=Stein>{{cite journal |last1=Stein |first1=Lawrence |date=1983 |title=The Chemistry of Radon |journal=Radiochimica Acta |volume=32 |issue=1–3 |pages=163–171 |doi=10.1524/ract.1983.32.13.163|s2cid=100225806 }}</ref><ref>{{cite journal |last1=Liebman |first1=Joel F. |date=1975 |title=Conceptual Problems in Noble Gas and Fluorine Chemistry, II: The Nonexistence of Radon Tetrafluoride |journal=Inorg. Nucl. Chem. Lett. |volume=11 |issue=10 |pages=683–685 |doi=10.1016/0020-1650(75)80185-1}}</ref><ref>{{cite journal |last1=Seppelt |first1=Konrad |date=2015 |title=Molecular Hexafluorides |journal=Chemical Reviews |volume=115 |issue=2 |pages=1296–1306 |doi=10.1021/cr5001783|pmid=25418862 }}</ref>

Krypton is less reactive than xenon, but several compounds have been reported with krypton in the [[oxidation state]] of +2.<ref name=Ngcomp/> [[Krypton difluoride]] is the most notable and easily characterized. Under extreme conditions, krypton reacts with fluorine to form KrF<sub>2</sub> according to the following equation:

::Kr + F<sub>2</sub> → KrF<sub>2</sub>

Compounds in which krypton forms a single bond to nitrogen and oxygen have also been characterized,<ref>{{cite journal|doi=10.1016/S0010-8545(02)00202-3|title=The chemistry of krypton|year=2002|author=Lehmann, J|journal=Coordination Chemistry Reviews|volume=233–234|pages=1–39}}</ref> but are only stable below {{convert|-60|C}} and {{convert|-90|C}} respectively.<ref name=Ngcomp/>

Krypton atoms chemically bound to other nonmetals (hydrogen, chlorine, carbon) as well as some late [[transition metal]]s (copper, silver, gold) have also been observed, but only either at low temperatures in noble gas matrices, or in supersonic noble gas jets.<ref name=Ngcomp/> Similar conditions were used to obtain the first few compounds of argon in 2000, such as [[argon fluorohydride]] (HArF), and some bound to the late transition metals copper, silver, and gold.<ref name=Ngcomp/> As of 2007, no stable neutral molecules involving covalently bound helium or neon are known.<ref name=Ngcomp/>

Extrapolation from periodic trends predict that oganesson should be the most reactive of the noble gases; more sophisticated theoretical treatments indicate greater reactivity than such extrapolations suggest, to the point where the applicability of the descriptor "noble gas" has been questioned.<ref>{{cite journal |last=Roth |first=Klaus |year=2017 |title=Ist das Element 118 ein Edelgas? |trans-title=Is Element 118 a Noble Gas? |journal=[[Chemie in unserer Zeit]] |language=German |volume=51 |issue=6 |pages=418–426 |doi=10.1002/ciuz.201700838}}<br />Translated into English by W. E. Russey and published in three parts in [[ChemViews Magazine]]:<br />{{cite magazine |last=Roth |first=Klaus |date=3 April 2018 |title=New Kids on the Table: Is Element 118 a Noble Gas? – Part 1 |url=https://www.chemistryviews.org/details/ezine/10907570/New_Kids_on_the_Table_Is_Element_118_a_Noble_Gas__Part_1.html |magazine=[[ChemViews Magazine]] |doi=10.1002/chemv.201800029}}<br />{{cite magazine |last=Roth |first=Klaus |date=1 May 2018 |title=New Kids on the Table: Is Element 118 a Noble Gas? – Part 2 |url=https://www.chemistryviews.org/details/ezine/11002036/New_Kids_on_the_Table_Is_Element_118_a_Noble_Gas__Part_2.html |magazine=[[ChemViews Magazine]] |doi=10.1002/chemv.201800033}}<br />{{cite magazine |last=Roth |first=Klaus |date=5 June 2018 |title=New Kids on the Table: Is Element 118 a Noble Gas? – Part 3 |url=https://www.chemistryviews.org/details/ezine/11046703/New_Kids_on_the_Table_Is_Element_118_a_Noble_Gas__Part_3.html |magazine=[[ChemViews Magazine]] |doi=10.1002/chemv.201800046}}</ref> Oganesson is expected to be rather like [[silicon]] or [[tin]] in group 14:<ref name=primefan>{{cite web |url=http://www.primefan.ru/stuff/chem/ptable/ptable.pdf |title=Есть ли граница у таблицы Менделеева? |trans-title=Is there a boundary to the Mendeleev table? |last=Kulsha |first=A. V. |website=www.primefan.ru |access-date=8 September 2018 |language=ru}}</ref> a reactive element with a common +4 and a less common +2 state,<ref name="BFricke">{{Cite journal |last1=Fricke |first1=Burkhard |year=1975 |title=Superheavy elements: a prediction of their chemical and physical properties |journal=Recent Impact of Physics on Inorganic Chemistry |volume=21 |pages=[https://archive.org/details/recentimpactofph0000unse/page/89 89–144] |doi=10.1007/BFb0116498 |url=https://archive.org/details/recentimpactofph0000unse/page/89 |access-date=4 October 2013 |series=Structure and Bonding |isbn=978-3-540-07109-9}}</ref><ref>[http://www.primefan.ru/stuff/chem/front2019.png Russian periodic table poster] by A. V. Kulsha and T. A. Kolevich</ref> which at room temperature and pressure is not a gas but rather a solid semiconductor. Empirical / experimental testing will be required to validate these predictions.<ref name=og/><ref name=semiconductor>{{cite journal |last1=Mewes |first1=Jan-Michael |last2=Smits |first2=Odile Rosette |first3=Paul |last3=Jerabek |first4=Peter |last4=Schwerdtfeger |date=25 July 2019 |title=Oganesson is a Semiconductor: On the Relativistic Band-Gap Narrowing in the Heaviest Noble-Gas Solids |journal=Angewandte Chemie |volume=58 |issue=40 |pages=14260–14264 |doi=10.1002/anie.201908327 |pmid=31343819 |pmc=6790653 }}</ref> (On the other hand, [[flerovium]], despite being in group 14, is predicted to be unusually volatile, which suggests noble gas-like properties.)<ref>{{Cite journal |last=Kratz |first=J. V. |date=2012-08-01 |title=The impact of the properties of the heaviest elements on the chemical and physical sciences |journal=Radiochimica Acta |language=en |volume=100 |issue=8–9 |pages=569–578 |doi=10.1524/ract.2012.1963 |s2cid=97915854 |issn=2193-3405|doi-access=free }}</ref><ref>{{Cite web |title=Indication for a volatile element 114 |url=http://doc.rero.ch/record/290779/files/ract.2010.1705.pdf |website=doc.rero.ch}}</ref>

The noble gases—including helium—can form stable [[molecular ion]]s in the gas phase. The simplest is the [[Helium hydride ion|helium hydride molecular ion]], HeH<sup>+</sup>, discovered in 1925.<ref>{{cite journal |author1=Hogness, T. R. |author2=Lunn, E. G. |title=The Ionization of Hydrogen by Electron Impact as Interpreted by Positive Ray Analysis |journal=Physical Review |year=1925 |volume=26 |issue=1 |pages=44–55 |doi=10.1103/PhysRev.26.44 |bibcode=1925PhRv...26...44H}}</ref> Because it is composed of the two most abundant elements in the universe, hydrogen and helium, it was believed to occur naturally in the [[interstellar medium]], and it was finally detected in April 2019 using the airborne [[Stratospheric Observatory for Infrared Astronomy|SOFIA telescope]]. In addition to these ions, there are many known neutral [[excimer]]s of the noble gases. These are compounds such as ArF and KrF that are stable only when in an [[Excited state|excited electronic state]]; some of them find application in [[excimer laser]]s.

In addition to the compounds where a noble gas atom is involved in a [[covalent bond]], noble gases also form [[non-covalent]] compounds. The [[clathrate]]s, first described in 1949,<ref>{{cite journal |title=An Inert Gas Compound |author1=Powell, H. M. |author2=Guter, M. |name-list-style=amp |journal=Nature |volume=164 |pages=240–241 |year=1949 |doi=10.1038/164240b0 |pmid=18135950 |issue=4162|bibcode= 1949Natur.164..240P |s2cid=4134617 }}</ref> consist of a noble gas atom trapped within cavities of [[crystal lattice]]s of certain organic and inorganic substances. The essential condition for their formation is that the guest (noble gas) atoms must be of appropriate size to fit in the cavities of the host crystal lattice. For instance, argon, krypton, and xenon form clathrates with [[hydroquinone]], but helium and neon do not because they are too small or insufficiently [[Polarizability|polarizable]] to be retained.<ref>{{harvnb|Greenwood|1997|p=893}}</ref> Neon, argon, krypton, and xenon also form clathrate hydrates, where the noble gas is trapped in ice.<ref>{{cite journal |author=Dyadin, Yuri A.|doi=10.1070/MC1999v009n05ABEH001104 |title=Clathrate hydrates of hydrogen and neon |journal=Mendeleev Communications |volume=9 |issue=5 |year=1999 |pages=209–210|display-authors=etal}}</ref>

[[File:Endohedral fullerene.png|thumb|An endohedral fullerene compound containing a noble gas atom|alt=A skeletal structure of buckminsterfullerene with an extra atom in its center.]]

Noble gases can form [[Endohedral fullerenes#Non-metal doped fullerenes|endohedral fullerene]] compounds, in which the noble gas atom is trapped inside a [[fullerene]] molecule. In 1993, it was discovered that when {{chem|C|60}}, a spherical molecule consisting of 60&nbsp;[[carbon]]&nbsp;atoms, is exposed to noble gases at high pressure, [[Complex (chemistry)|complex]]es such as {{chem|He@C|60}} can be formed (the ''@'' notation indicates He is contained inside {{chem|C|60}} but not covalently bound to it).<ref>{{cite journal|title=Stable compounds of helium and neon. He@C60 and Ne@C60|author1=Saunders, M. |author2=Jiménez-Vázquez, H. A. |author3=Cross, R. J. |author4=Poreda, R. J. |journal=[[Science (journal)|Science]]|year=1993|volume=259|pages=1428–1430|doi=10.1126/science.259.5100.1428|pmid=17801275|issue=5100|bibcode= 1993Sci...259.1428S |s2cid=41794612 }}</ref> As of 2008, endohedral complexes with helium, neon, argon, krypton, and xenon have been created.<ref>{{cite journal|title=Incorporation of helium, neon, argon, krypton, and xenon into fullerenes using high pressure|author1=Saunders, Martin |author2=Jimenez-Vazquez, Hugo A. |author3=Cross, R. James |author4=Mroczkowski, Stanley |author5=Gross, Michael L. |author6=Giblin, Daryl E. |author7=Poreda, Robert J. |journal=[[J. Am. Chem. Soc.]]|year=1994|volume=116|issue=5|pages=2193–2194|doi=10.1021/ja00084a089|bibcode=1994JAChS.116.2193S }}</ref> These compounds have found use in the study of the structure and reactivity of fullerenes by means of the [[nuclear magnetic resonance]] of the noble gas atom.<ref>{{cite journal|last1=Frunzi|first1=Michael|last2=Cross|first2=R. Jame|last3=Saunders|first3=Martin|title=Effect of Xenon on Fullerene Reactions|journal=[[Journal of the American Chemical Society]]|year=2007|volume=129|doi=10.1021/ja075568n|pages=13343–13346|pmid=17924634|issue=43|bibcode=2007JAChS.12913343F |url=https://figshare.com/articles/journal_contribution/2977702}}</ref>

[[File:XeF2.svg|300px|thumb|left|Bonding in {{chem|XeF|2}} according to the 3-center-4-electron bond model|alt=Schematic illustration of bonding and antibonding orbitals (see text)]]
Noble gas compounds such as [[xenon difluoride]] ({{chem|XeF|2}}) are considered to be [[hypervalent]] because they violate the [[octet rule]]. Bonding in such compounds can be explained using a [[three-center four-electron bond]] model.<ref>{{harvnb|Greenwood|1997|p=897}}</ref><ref>{{harvnb|Weinhold|2005|pp=275–306}}</ref> This model, first proposed in 1951, considers bonding of three collinear atoms. For example, bonding in {{chem|XeF|2}} is described by a set of three [[molecular orbital]]s (MOs) derived from [[p-orbital]]s on each atom. Bonding results from the combination of a filled p-orbital from Xe with one half-filled p-orbital from each [[fluorine|F]] atom, resulting in a filled bonding orbital, a filled non-bonding orbital, and an empty [[antibonding]] orbital. The [[highest occupied molecular orbital]] is localized on the two terminal atoms. This represents a localization of charge that is facilitated by the high electronegativity of fluorine.<ref>{{cite journal|last=Pimentel|first=G. C.|title= The Bonding of Trihalide and Bifluoride Ions by the Molecular Orbital Method|year=1951|issue=4|pages=446–448|doi=10.1063/1.1748245|journal=The Journal of Chemical Physics|volume=19|bibcode= 1951JChPh..19..446P }}</ref>

The chemistry of the heavier noble gases, krypton and xenon, are well established. The chemistry of the lighter ones, argon and helium, is still at an early stage, while a neon compound is yet to be identified.
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