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In chemistry, noble gas compounds are chemical compounds that include an Chemical element from the , 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: the relatively reactive krypton (ionisation energy 14.0 Electronvolt), 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 using spectroscopic techniques, but only when frozen into a noble gas matrix at temperatures of or lower, in supersonic jets of noble gas, or under extremely high pressures with metals.
The heavier noble gases have more than the lighter ones. Hence, the outermost electrons are subject to a shielding effect from the inner electrons that makes them more easily ionization, since they are less strongly attracted to the positively-charged atomic 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.
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 () and xenon hexafluoride (), speculated that might exist as an unstable compound, and suggested that xenic acid would form perxenate salts. These predictions proved quite accurate, although subsequent predictions for indicated that it would be not only thermodynamics unstable, but kinetically unstable. As of 2022, has not been made, although the octafluoroxenate(VI) anion () has been observed.
By 1960, no compound with a covalently bound noble gas atom had yet been synthesized. (1999). 9780138418915, Prentice Hall. ISBN 9780138418915 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 to dioxygenyl. As the ionisation energy of to (1165 kJ mol−1) is nearly equal to the ionisation energy of Xe to (1170 kJ mol−1), he tried the reaction of Xe with . This yielded a crystalline product, xenon hexafluoroplatinate, whose formula was proposed to be .
It was later shown that the compound is actually more complex, containing both and . Nonetheless, this was the first real compound of any noble gas.
The first binary compound noble gas compounds were reported later in 1962. Bartlett synthesized xenon tetrafluoride () by subjecting a mixture of xenon and fluorine to high temperature. Rudolf Hoppe, among other groups, synthesized xenon difluoride () by the reaction of the elements.
Following the first successful synthesis of xenon compounds, synthesis of krypton difluoride () was reported in 1963.
In terms of other halide reactivity, short-lived of noble gas halides such as xenon dichloride or XeCl are prepared in situ, and are used in the function of .
Recently, xenon has been shown to produce a wide variety of compounds of the type where n is 1, 2 or 3 and X is any electronegative group, such as , Triflidic acid, , Bistriflimide, Teflate, , 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.
The compound contains a Xe–Xe bond, which is the longest element-element bond known (308.71 pm = 3.0871 Angstrom). Short-lived of are reported to exist as a part of the function of .
reacts with strong [[Lewis acid]]s to form salts of the and [[cation]]s. The preparation of reported by Grosse in 1963, using the Claasen method, was subsequently shown to be a mistaken identification.
Krypton compounds with other than Kr–F bonds (compounds with atoms other than fluorine) have also been described. reacts with to produce the unstable compound, , with a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation , produced by the reaction of with below −50 °C.
There is a possibility that a solid salt of could be prepared with or anions. (2005). 9783540135340 ISBN 9783540135340
All known oganesson isotopes have even shorter half-lives in the millisecond range and no compounds are known yet, (2013). 9783642374661, Springer Science & Business Media. ISBN 9783642374661 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.
Helium-nitrogen () crystals have been grown at room temperature at pressures ca. 10 GPa in a diamond anvil cell. Solid argon-hydrogen clathrate () has the same crystal structure as the Laves phase. It forms at pressures between 4.3 and 220 GPa, though Raman measurements suggest that the molecules in dissociate above 175 GPa. A similar 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 xenon atoms form dimers inside solid hydrogen.
Xenon is known to function as a metal ligand. In addition to the charged [tetraxenonogold(II)|[AuXe42+]], xenon, krypton, and argon all reversibly bind to gaseous metal carbonyl, 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.
Also, compounds such as and were reported to have been formed by electron bombardment, but recent research has shown that these are probably the result of He being on the surface of the metal; therefore, these compounds cannot truly be considered chemical compounds.
Stable salts of xenon containing very high proportions of fluorine by weight (such as tetrafluoroammonium heptafluoroxenate(VI), , and the related tetrafluoroammonium octafluoroxenate(VI) ), have been developed as highly energetic oxidisers for use as propellants in rocketry. Christe, Karl O., Wilson, William W. Perfluoroammonium salt of heptafluoroxenon anion. , June 24, 1982
Xenon fluorides are good fluorinating agents.
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. (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.) 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 , while 133Xe clathrate provides a useful source of .
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