Noble gas compound 435089 220646851 2008-06-20T21:43:40Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. '''Noble gas compounds''' are [[chemical compound]]s that include an [[Chemical element|element]] from [[Periodic table group|Group]] 18<!-- not "8"; do not change without consulting the [[Noble gas]] page--> of the [[periodic table]], the [[noble gas]]es. ==History and background== Until the 20th century it was believed that the noble gases could not form compounds due to their full [[valence shell]] of [[electrons]] that rendered them very chemically stable and unreactive. All noble gases have full s and p outer [[electron shell]]s (i.e. 8 outer shell electrons, except [[helium]], which has 2, but is nonetheless stable), and so do not form [[chemical compound]]s easily. Because of their high [[ionization energy]] and almost zero [[electron affinity]], they were not expected to be reactive at all. In 1933, however, [[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]] and [[xenon hexafluoride]] ([[Xenon|Xe]][[Fluorine|F]]<sub>6</sub>), speculated that XeF<sub>8</sub> might exist as an unstable compound, and suggested that [[xenic acid]] would form [[perxenate]] salts.<ref>{{cite journal | title = The Formulas of Antimonic Acid and the Antimonates | author = Linus Pauling | journal = J. Am. Chem. Soc. | volume = 55, | issue = 5 | pages = 1895–1900 | month = June | year = 1933 | url = | doi = 10.1021/ja01332a016}}</ref><ref name="Holloway">{{cite book | last = Holloway | first = John H. | year = 1968 | title = Noble-Gas Chemistry | publisher = Methuen | location = London }}</ref> These predictions proved quite accurate, except that XeF<sub>8</sub> is now predicted to be not only thermodynamically unstable, but kinematically unstable,<ref>{{cite journal | last = Seppelt | first = Konrad | year = 1979 | month = June | title = Recent developments in the Chemistry of Some Electronegative Elements | journal = Accounts of Chemical Research | volume = 12 | pages = 211&ndash;216 | doi = 10.1021/ar50138a004 }}</ref> and as of 2006 has not been made. The heavier noble gases have more electron shells than those near the top. Hence, the outermost electrons experience a [[shielding effect]] from the inner electrons that makes it easier to [[ionize]] them since they are less strongly attracted to the positively-charged [[atomic nucleus|nucleus]]. This results in an ionization energy low enough to form stable compounds with the most [[electronegativity|electronegative]] elements, fluorine and oxygen. ==Pre-1962 compounds== Prior to 1962, the only isolated compounds of noble gases were [[Clathrate compound|clathrates]] (including clathrate [[hydrate]]s). Other compounds such as [[Complex (chemistry)|coordination compounds]] were observed only by spectroscopic means.<ref name="Holloway" /> ===Clathrates=== 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. The essential condition for their formation is that the guest (noble gas) atoms should be of appropriate size to fit in the cavities of the host crystal lattice. For instance, Ar, Kr and Xe can form clathrates with β-quinol, but He and Ne cannot fit because they are too small. 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. In addition,<sup>85</sup>Kr clathrate provides a safe source of [[beta particle]]s, while <sup>133</sup>Xe clathrate provides a useful source of [[gamma ray]]s. ===Coordination compounds=== Coordination compounds such as Ar·BF<sub>3</sub> were postulated to exist at low temperatures, but have never been confirmed. Also, compounds such as WHe<sub>2</sub> and HgHe<sub>2</sub> were reported to have been formed by electron bombardment, but recent research has shown that He is probably adsorbed on the surface of the metal, hence these compounds cannot be called true chemical compounds. ===Hydrates=== Hydrates are formed by compressing the noble gases in water. 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 Xe·6H<sub>2</sub>O is the most stable hydrate. The existence of these compounds has, however, been disputed in recent years.{{Fact|date=February 2007}} ==True noble gas compounds== In 1962, [[Neil Bartlett]] noticed that the highly oxidising compound [[platinum hexafluoride]] ionised [[oxygen|O<sub>2</sub>]] to [[dioxygenyl|O<sub>2</sub><sup>+</sup>]]. As the ionisation energy of O<sub>2</sub> to O<sub>2</sub><sup>+</sup> (1165&nbsp;kJ&nbsp;mol<sup>–1</sup>) is nearly equal to the ionisation energy of Xe to Xe<sup>+</sup> (1170&nbsp;kJ&nbsp;mol<sup>–1</sup>), he tried the reaction of Xe with PtF<sub>6</sub>. This yielded a crystalline product [[xenon hexafluoroplatinate]], whose formula was proposed to be Xe<sup>+</sup>[PtF<sub>6</sub>]<sup>–</sup>.<ref name="Holloway" /><ref name="bartlett">{{cite journal | title = Xenon hexafluoroplatinate Xe<sup>+</sup>[PtF<sub>6</sub>]<sup>–</sup> | author = Bartlett, N. | journal = Proceedings of the Chemical Society of London | volume = | issue = 6 | pages = 218 | year = 1962 | url = | doi = 10.1039/PS9620000197}}</ref> It was later shown{{Fact|date=January 2008}} that the compound is actually more complex, containing both XeFPtF<sub>6</sub> and XeFPt<sub>2</sub>F<sub>11</sub>. This was the first real compound of any noble gas. Later in 1962 the first simple (two-element) noble gas compound (xenon tetrafluoride) was synthesized by Howard Claassen by subjecting xenon and fluorine to a high temperature.<ref>{{cite journal |author= Claassen, H. H.; Selig, H.; Malm, J. G. |title= Xenon Tetrafluoride |journal= [[J. Am. Chem. Soc.]] |volume= 84 |issue= 18 |pages= 3593 |year= 1962 |doi= 10.1021/ja00877a042 }}</ref> In recent years, several compounds of noble gases, particularly xenon, have been prepared. Among these are the xenon fluorides ([[xenon difluoride|XeF<sub>2</sub>]], [[xenon tetrafluoride|XeF<sub>4</sub>]], [[xenon hexafluoride|XeF<sub>6</sub>]]), oxyfluorides (XeOF<sub>2</sub>, XeOF<sub>4</sub>, XeO<sub>2</sub>F<sub>2</sub>, XeO<sub>3</sub>F<sub>2</sub>, XeO<sub>2</sub>F<sub>4</sub>) and oxides ([[xenon trioxide|XeO<sub>3</sub>]] and [[xenon tetroxide|XeO<sub>4</sub>]]). Xenon difluoride can be produced by the simple exposure of Xe and F<sub>2</sub> gases to sunlight; while the mixing of the two gases had been tried over 50 years before in an attempt to produce a reaction, nobody had thought to simply expose the mixture to sunlight. Radon has reacted with fluorine to form [[radon fluoride|RnF<sub>2</sub>]], which glows with a yellow light in the solid state. Krypton is able to react with fluorine to form [[krypton fluoride|KrF<sub>2</sub>]], and short-lived [[excimer]]s of Xe<sub>2</sub> and noble gas [[halides]] such as [[xenon chloride|XeCl<sub>2</sub>]] are used in [[excimer laser]]s. The discovery of [[argon fluoride|ArF<sub>2</sub>]] was announced in 2003{{Fact|date=June 2008}} but so far is not confirmed. No conventional compounds of He or Ne are known. Recently xenon has been shown to produce a wide variety of compounds of the type XeO<sub>x</sub>Y<sub>2</sub> where x is 1,2 or 3 and Y is any electronegative group, such as CF<sub>3</sub>, C(SO<sub>2</sub>CF<sub>3</sub>)<sub>3</sub>, N(SO<sub>2</sub>F)<sub>2</sub>, N(SO<sub>2</sub>CF<sub>3</sub>)<sub>2</sub>, OTeF<sub>5</sub>, O(IO<sub>2</sub>F<sub>2</sub>) etc. The range of compounds is impressive, running into the thousands and involving bonds between xenon and oxygen, nitrogen, carbon, and even gold, as well as perxenic acid, several halides, and complex ions; a range of compounds seen in the neighbouring element [[iodine]]. The compound Xe<sub>2</sub>Sb<sub>2</sub>F<sub>11</sub> contains a Xe–Xe bond, the longest element-element bond known (308.71&nbsp;pm). ==Fullerene compounds== [[Image:Endohedral fullerene.png|thumb|right]]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 C<sub>60</sub> is exposed to a pressure of around 3 [[bar (unit)|bar]] of He or Ne, the complexes He@C<sub>60</sub> and Ne@C<sub>60</sub> are formed.<ref>{{cite journal|title=Stable compounds of helium and neon. He@C60 and Ne@C60|author=M. Saunders, H. A. Jiménez-Vázquez, R. J. Cross, and R. J. Poreda|journal=[[Science (journal)|Science]]|year=1993|volume=259|pages=1428–1430|doi=10.1126/science.259.5100.1428|pmid=17801275}}</ref> Under these conditions, only about one out of every 650,000 C<sub>60</sub> 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 He@C<sub>60</sub>.<ref>{{cite journal|title=Incorporation of helium, neon, argon, krypton, and xenon into fullerenes using high pressure|author=Martin Saunders, Hugo A. Jimenez-Vazquez, R. James Cross, Stanley Mroczkowski, Michael L. Gross, Daryl E. Giblin, and Robert J. Poreda|journal=[[J. Am. Chem. Soc.]]|year=1994|volume=116|issue=5|pages=2193–2194|doi=10.1021/ja00084a089}}</ref> ==Applications== 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: the xenon is simply liberated as a gas. It is rivalled only by [[ozone]] in this respect.<ref name="Holloway" /> The [[perxenate]]s are even more powerful oxidising agents, and the xenon fluorides are good fluorinating agents. 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" /> ==References== <references /> ==Resources== *http://www.chemsoc.org/exemplarchem/entries/2001/robson/raregascompounds.htm *http://www.nationmaster.com/encyclopedia/Noble-gas *http://www.webelements.com/webelements/elements/ [[Category:Noble gas compounds| ]] [[de:Edelgasverbindungen]] [[fi:Jalokaasuyhdiste]] [[zh:稀有气体化合物]]