Noble gas compound
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Noble gas compounds are (unstable) chemical compounds that include an element from column 18 of the periodic table, the noble gases.
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History
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 shells (i.e. 8 outer shell electrons, except helium, with 2, but is nonetheless stable), and so do not form chemical compounds easily. Because of their high ionisation energy and almost zero electron affinity, they were not expected to be reactive at all.
However, in 1933, Linus Pauling predicted that the noble gases near the bottom of the periodic table would be able to form compounds with fluorine. This is because noble gases near the bottom of the table have more valence 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 nucleus. Additionally fluorine has the highest electronegativity of all the elements, making it the most reactive element.
Pre-1962 Compounds
Prior to 1962, the main compounds of noble gas known were the hydrates and clathrates. In addition, coordination compounds, and compounds formed under excited conditions were also reported to be formed.
Hydrates are formed by compressing the noble gases in water. It is believed that the water molecule (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·6H2O is the most stable hydrate. The existence of these compounds has, however, been disputed in recent years.
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 not He or Ne.
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,85Kr clathrate provides a safe source of β-radiations, while 133Xe clathrate provides a useful source of γ-radiations.
Coordination compounds such as Ar·BF3 were postulated to exist at low temperatures, but have never been confirmed. Also, compounds such as WHe2, HgHe2, and HgHe2 were reported to have been formed by electron bombardment, but recent research has shown that He is probably absorbed on the surface of the metal, hence these compounds cannot be called true chemical compounds.
True Noble gas compounds
In 1962, Neil Bartlett noticed that a highly oxidising compound PtF6 ionised O2 to O2+. As the ionisation energy of this, at 1165 kJmol-1 is close to the ionisation energy of 1170 kJmol-1 required for Xe to Xe+ transition, he tried the reaction of Xe with PtF6, which yielded a crystalline solid with the formula Xe+[PtF6]-. This was the first real compound of any noble gas.
In recent years, several compounds of noble gases, particularly xenon, have been prepared. Among these are the xenon fluorides(XeF2, XeF4, XeF6), oxyfluorides(XeOF2, XeOF4, XeO2F2, XeO3F2, XeO2F4) and oxides (XeO3, XeO4).
Radon has reacted with fluorine to form radon fluoride (RnF2), which glows with a yellow light in the solid state. Krypton is able to react with fluorine to form krypton monoflouride KrF2, and short-lived excimers of Xe2 and noble gas halides such as xenon chloride (XeCl2) are used in excimer lasers. The discovery of argon fluoride (ArF2) was announced in 2003.
In 2002, compounds were discovered where uranium forms molecules with argon, krypton, or xenon. This suggests that the noble gases may be able to form compounds with other metals too.
Noble gas compounds have found use as fluorinating agents, oxidising agents, etc.