Noble gas compound

Noble gas compounds are chemical compounds that include an element from the noble gases, group 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 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 (at least in principle for the highly radioactive radon), 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 40 K 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.[1][2]

History and background

When the family of noble gases was first identified at the end of the nineteenth century, none of them was observed to form any compounds and 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 (KrF6) and xenon hexafluoride (XeF6), speculated that XeF8 might exist as an unstable compound, and suggested that xenic acid would form perxenate salts.[3][4] These predictions proved quite accurate, although subsequent predictions for XeF8 indicated that it would be not only thermodynamically unstable, but kinetically unstable.[5] As of 2013, XeF8 has not been made, although the octafluoroxenate(VI) anion (8) has been observed.

By 1960, no compound with a covalently bound noble gas atom had yet been synthesized.[6] 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 O2 to 2. As the ionisation energy of O2 to O+
2
(1165 kJ mol−1) is nearly equal to the ionisation energy of Xe to Xe+ (1170 kJ mol−1), he tried the reaction of Xe with PtF6. This yielded a crystalline product, xenon hexafluoroplatinate, whose formula was proposed to be Xe+
[PtF
6
]
.[4][7] It was later shown that the compound is actually more complex, containing both XeFPtF5 and XeFPt2F11.[8] 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 (XeF4) by subjecting a mixture of xenon and fluorine to high temperature.[9] Rudolf Hoppe, among other groups, synthesized xenon difluoride (XeF2) by the reaction of the elements.[10]

Following the first successful synthesis of xenon compounds, synthesis of krypton difluoride (KrF
2
) was reported in 1963.[11]

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