Noble gas compound
In chemistry, noble gas compounds are chemical compounds that include an element from the noble gases, group 8 or 18 of the periodic table. Although the noble gases are generally unreactive elements, many such compounds have been observed, particularly involving the element xenon.
From the standpoint of chemistry, the noble gases may be divided into two groups:Template:Citation needed the relatively reactive krypton (ionisation energy 14.0 eV), xenon (12.1 eV), and radon (10.7 eV) on one side, and the very unreactive argon (15.8 eV), neon (21.6 eV), and helium (24.6 eV) on the other. Consistent with this classification, Kr, Xe, and Rn form compounds that can be isolated in bulk at or near standard temperature and pressure, whereas He, Ne, Ar have been observed to form true chemical bonds using spectroscopic techniques, but only when frozen into a noble gas matrix at temperatures of Template:Convert or lower, in supersonic jets of noble gas, or under extremely high pressures with metals.
The heavier noble gases have more electron shells than the lighter ones. Hence, the outermost electrons are subject to a shielding effect from the inner electrons that makes them more easily ionized, since they are less strongly attracted to the positively-charged nucleus. This results in an ionization energy low enough to form stable compounds with the most electronegative elements, fluorine and oxygen, and even with less electronegative elements such as nitrogen and carbon under certain circumstances.<ref name="pmid17256847">Template:Cite journal</ref><ref name="pmid18407626">Template:Cite journal</ref>
History and backgroundEdit
When the family of noble gases was first identified at the end of the nineteenth century, none of them were observed to form any compounds and so it was initially believed that they were all inert gases (as they were then known) which could not form compounds. With the development of atomic theory in the early twentieth century, their inertness was ascribed to a full valence shell of electrons which render them very chemically stable and nonreactive. All noble gases have full s and p outer electron shells (except helium, which has no p sublevel), and so do not form chemical compounds easily. Their high ionization energy and almost zero electron affinity explain their non-reactivity.
In 1933, Linus Pauling predicted that the heavier noble gases would be able to form compounds with fluorine and oxygen. Specifically, he predicted the existence of krypton hexafluoride (Template:Chem2) and xenon hexafluoride (Template:Chem2), speculated that Template:Chem2 might exist as an unstable compound, and suggested that xenic acid would form perxenate salts.<ref>Template:Cite journal</ref><ref name="Holloway">Template:Cite book</ref> These predictions proved quite accurate, although subsequent predictions for Template:Chem2 indicated that it would be not only thermodynamically unstable, but kinetically unstable.<ref>Template:Cite journal</ref> As of 2022, Template:Chem2 has not been made, although the octafluoroxenate(VI) anion ([[Nitrosonium octafluoroxenate(VI)|Template:Chem2]]) has been observed.
By 1960, no compound with a covalently bound noble gas atom had yet been synthesized.<ref>Template:Cite book</ref> The first published report, in June 1962, of a noble gas compound was by Neil Bartlett, who noticed that the highly oxidising compound platinum hexafluoride ionised [[oxygen|Template:Chem2]] to [[dioxygenyl|Template:Chem2]]. As the ionisation energy of Template:Chem2 to Template:Chem2 (1165 kJ mol−1) is nearly equal to the ionisation energy of Xe to Template:Chem2 (1170 kJ mol−1), he tried the reaction of Xe with Template:Chem2. This yielded a crystalline product, xenon hexafluoroplatinate, whose formula was proposed to be Template:Chem2.<ref name="Holloway" /><ref name="bartlett">Template:Cite journal</ref> It was later shown that the compound is actually more complex, containing both Template:Chem2 and Template:Chem2.<ref name="grahm">Template:Cite journal</ref> Nonetheless, this was the first real compound of any noble gas.
The first binary noble gas compounds were reported later in 1962. Bartlett synthesized xenon tetrafluoride (Template:Chem2) by subjecting a mixture of xenon and fluorine to high temperature.<ref>Template:Cite journal</ref> Rudolf Hoppe, among other groups, synthesized xenon difluoride (Template:Chem2) by the reaction of the elements.<ref>Template:Cite journal</ref>
Following the first successful synthesis of xenon compounds, synthesis of krypton difluoride (Template:Chem2) was reported in 1963.<ref name="Lehmann">Template:Cite journal</ref>
True noble gas compoundsEdit
In this section, the non-radioactive noble gases are considered in decreasing order of atomic weight, which generally reflects the priority of their discovery, and the breadth of available information for these compounds. The radioactive elements radon and oganesson are harder to study and are considered at the end of the section.
Xenon compoundsEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} After the initial 1962 studies on [[Xenon tetrafluoride|Template:Chem2]] and [[Xenon difluoride|Template:Chem2]], xenon compounds that have been synthesized include other fluorides ([[xenon hexafluoride|Template:Chem2]]), oxyfluorides ([[Xenon oxydifluoride|Template:Chem2]], [[Xenon oxytetrafluoride|Template:Chem2]], [[Xenon dioxydifluoride|Template:Chem2]], Template:Chem2, Template:Chem2) and oxides ([[xenon dioxide|Template:Chem2]], [[xenon trioxide|Template:Chem2]] and [[xenon tetroxide|Template:Chem2]]). Xenon fluorides react with several other fluorides to form fluoroxenates, such as sodium octafluoroxenate(VI) (Template:Chem2),Template:Citation needed and fluoroxenonium salts, such as trifluoroxenonium hexafluoroantimonate (Template:Chem2).<ref>Template:Cite journal</ref>
In terms of other halide reactivity, short-lived excimers of noble gas halides such as [[xenon dichloride|Template:Chem2]] or XeCl are prepared in situ, and are used in the function of excimer lasers.<ref>Template:Cite journal</ref>
Recently,Template:When xenon has been shown to produce a wide variety of compounds of the type Template:Chem2 where n is 1, 2 or 3 and X is any electronegative group, such as Template:Chem2, [[Triflidic acid|Template:Chem2]], Template:Chem2, [[Bistriflimide|Template:Chem2]], [[Teflate|Template:Chem2]], Template:Chem2, etc.; the range of compounds is impressive, similar to that seen with the neighbouring element iodine, running into the thousands and involving bonds between xenon and oxygen, nitrogen, carbon, boron and even gold, as well as perxenic acid, several halides, and complex ions.Template:Citation needed
The compound Template:Chem2 contains a Xe–Xe bond, which is the longest element-element bond known (308.71 pm = 3.0871 Å).<ref>Template:Cite journal</ref> Short-lived excimers of Template:Chem2 are reported to exist as a part of the function of excimer lasers.Template:Citation needed
Krypton compoundsEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Krypton gas reacts with fluorine gas under extreme forcing conditions, forming [[Krypton difluoride|Template:Chem2]] according to the following equation:
Template:Chem2 reacts with strong Lewis acids to form salts of the Template:Chem2 and Template:Chem2 cations.<ref name="Lehmann"/> The preparation of Template:Chem2 reported by Grosse in 1963, using the Claasen method, was subsequently shown to be a mistaken identification.<ref>Template:Cite journal</ref>
Krypton compounds with other than Kr–F bonds (compounds with atoms other than fluorine) have also been described. Template:Chem2 reacts with Template:Chem2 to produce the unstable compound, Template:Chem2, with a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation Template:Chem2, produced by the reaction of Template:Chem2 with Template:Chem2 below −50 °C.<ref>Template:Cite book</ref>
Argon compoundsEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:Expand section The discovery of HArF was announced in 2000.<ref>Template:Cite journal</ref><ref name=Bochenkova>Template:Cite journal</ref> The compound can exist in low temperature argon matrices for experimental studies, and it has also been studied computationally.<ref name=Bochenkova/> Argon hydride ion Template:Chem2 was obtained in the 1970s.<ref>Template:Cite journal</ref> This molecular ion has also been identified in the Crab nebula, based on the frequency of its light emissions.<ref>Template:Cite journal</ref>
There is a possibility that a solid salt of Template:Chem2 could be prepared with [[Hexafluoroantimonate|Template:Chem2]] or [[hexafluoroaurate|Template:Chem2]] anions.<ref name=frenking>Template:Cite journal</ref><ref>Template:Cite book</ref>
Neon and helium compoundsEdit
Template:Anchor {{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} The ions, Template:Chem2, Template:Chem2, Template:Chem2, and Template:Chem2 are known from optical and mass spectrometric studies. Neon also forms an unstable hydrate.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> There is some empirical and theoretical evidence for a few metastable helium compounds which may exist at very low temperatures or extreme pressures. The stable cation [[helium hydride ion|Template:Chem2]] was reported in 1925,<ref>Template:Cite journal</ref> but was not considered a true compound since it is not neutral and cannot be isolated. In 2016 scientists created the helium compound disodium helide (Template:Chem2) which was the first helium compound discovered.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Radon and oganesson compoundsEdit
Template:Anchor {{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Radon is not chemically inert, but its short half-life (3.8 days for 222Rn) and the high energy of its radioactivity make it difficult to investigate its only fluoride (Template:Chem2), its reported oxide (Template:Chem2), and their reaction products.<ref>Template:Cite journal</ref>
All known oganesson isotopes have even shorter half-lives in the millisecond range and no compounds are known yet,<ref name=Moody>Template:Cite book</ref> although some have been predicted theoretically. It is expected to be even more reactive than radon, more like a normal element than a noble gas in its chemistry.<ref name="BFricke">Template:Cite journal</ref>
Reports prior to xenon hexafluoroplatinate and xenon tetrafluorideEdit
ClathratesEdit
Prior to 1962, the only isolated compounds of noble gases were clathrates (including clathrate hydrates); other compounds such as coordination compounds were observed only by spectroscopic means.<ref name="Holloway" /> Clathrates (also known as cage compounds) are compounds of noble gases in which they are trapped within cavities of crystal lattices of certain organic and inorganic substances. Ar, Kr, Xe and Ne<ref>Template:Cite journal</ref> can form clathrates with crystalline hydroquinone. Kr and Xe can appear as guests in crystals of melanophlogite.<ref>Template:Cite journal</ref>
Helium-nitrogen (Template:Chem2) crystals have been grown at room temperature at pressures ca. 10 GPa in a diamond anvil cell.<ref>Template:Cite journal</ref> Solid argon-hydrogen clathrate (Template:Chem2) has the same crystal structure as the Template:Chem2 Laves phase. It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that the Template:Chem2 molecules in Template:Chem2 dissociate above 175 GPa. A similar Template:Chem2 solid forms at pressures above 5 GPa. It has a face-centered cubic structure where krypton octahedra are surrounded by randomly oriented hydrogen molecules. Meanwhile, in solid Template:Chem2 xenon atoms form dimers inside solid hydrogen.<ref name=KrH>Template:Cite journal</ref>
Coordination compoundsEdit
Coordination compounds such as Template:Chem2 have been postulated to exist at low temperatures, but have never been confirmed.Template:Citation needed
Xenon is known to function as a metal ligand. In addition to the charged [AuXe4]2+, xenon, krypton, and argon all reversibly bind to gaseous M(CO)5, where M=Cr, Mo, or W. P-block metals also bind noble gases: XeBeO has been observed spectroscopically and both XeBeS and FXeBO are predicted stable.<ref>Template:Cite journal</ref>
Also, compounds such as Template:Chem2 and Template:Chem2 were reported to have been formed by electron bombardment, but recent research has shown that these are probably the result of He being adsorbed on the surface of the metal; therefore, these compounds cannot truly be considered chemical compounds.Template:Citation needed
HydratesEdit
Hydrates are formed by compressing noble gases in water, where it is believed that the water molecule, a strong dipole, induces a weak dipole in the noble gas atoms, resulting in dipole-dipole interaction. Heavier atoms are more influenced than smaller ones, hence Template:Chem2 was reported to have been the most stable hydrate;<ref>Template:Cite journal Reprinted as Template:Cite book</ref> it has a melting point of 24 °C.<ref name="henderson2">Template:Cite book</ref> The deuterated version of this hydrate has also been produced.<ref>Template:Cite journal</ref>
Fullerene adductsEdit
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}
Noble gases can also form endohedral fullerene compounds where the noble gas atom is trapped inside a fullerene molecule. In 1993, it was discovered that when Template:Chem2 is exposed to a pressure of around 3 bar of He or Ne, the complexes Template:Chem2 and Template:Chem2 are formed.<ref>Template:Cite journal</ref> Under these conditions, only about one out of every 650,000 Template:Chem2 cages was doped with a helium atom; with higher pressures (3000 bar), it is possible to achieve a yield of up to 0.1%. Endohedral complexes with argon, krypton and xenon have also been obtained, as well as numerous adducts of Template:Chem2.<ref>Template:Cite journal</ref>
ApplicationsEdit
Most applications of noble gas compounds are either as oxidising agents or as a means to store noble gases in a dense form. Xenic acid is a valuable oxidising agent because it has no potential for introducing impurities—xenon is simply liberated as a gas—and so is rivalled only by ozone in this regard.<ref name="Holloway" /> The perxenates are even more powerful oxidizing agents.Template:Citation needed Xenon-based oxidants have also been used for synthesizing carbocations stable at room temperature, in [[sulfuryl chloride fluoride|Template:Chem2]] solution.<ref>Template:Cite journal</ref>Template:Primary source inline
Stable salts of xenon containing very high proportions of fluorine by weight (such as tetrafluoroammonium heptafluoroxenate(VI), Template:Chem2, and the related tetrafluoroammonium octafluoroxenate(VI) Template:Chem2), have been developed as highly energetic oxidisers for use as propellants in rocketry.<ref>Template:Cite journal</ref>Template:Primary source inline <ref>Christe, Karl O., Wilson, William W. Perfluoroammonium salt of heptafluoroxenon anion. Template:US Patent, June 24, 1982</ref>
Xenon fluorides are good fluorinating agents.<ref>Template:Cite journal</ref>
Clathrates have been used for separation of He and Ne from Ar, Kr, and Xe, and also for the transportation of Ar, Kr, and Xe.Template:Citation needed (For instance, radioactive isotopes of krypton and xenon are difficult to store and dispose, and compounds of these elements may be more easily handled than the gaseous forms.<ref name="Holloway" />) In addition, clathrates of radioisotopes may provide suitable formulations for experiments requiring sources of particular types of radiation; hence. 85Kr clathrate provides a safe source of beta particles, while 133Xe clathrate provides a useful source of gamma rays.<ref>Template:Cite journal</ref>
ReferencesEdit
ResourcesEdit
Template:Noble gas compounds Template:Chemical compounds by element