Occurrence

Occurrence: all the noble gases occur naturally in the atmosphere, generally as trace constituents (with the exception of argon, which at about 0.93% by volume is by far the most abundant of them and the third most abundant atmospheric gas overall); radon additionally arises continuously from the radioactive decay of heavier elements such as radium in the earth's crust.

Physical properties: moving down the group from helium to radon, both atomic radius and boiling point increase steadily, while the first ionisation energy decreases steadily; nonetheless, at any given period, the noble gas of that period has the highest ionisation energy of any element in the periodic table, a direct consequence of its completely filled valence shell. All the noble gases are monoatomic, colourless, odourless, tasteless, non-inflammable, non-metallic gases at ordinary temperature, and they show only a very small tendency to either gain or lose electrons, which is the fundamental reason for their chemical inertness.

Chemical properties: of all the noble gases, only xenon (and, to a much smaller extent, krypton) show any appreciable chemical reactivity, made possible because their outer electrons are held less tightly (lower ionisation energy) than those of the lighter noble gases. Xenon forms a series of fluorides by direct combination with fluorine gas under different conditions: with a nickel catalyst at 400 °C, xenon and one equivalent of fluorine give xenon difluoride (Xe + F2 -> XeF2); with excess fluorine (about 5:1) over nickel/acetone at 400 °C, xenon tetrafluoride forms (Xe + 2F2 -> XeF4); and with a large excess of fluorine (about 20:1) over nickel at 400 °C and 200 atm pressure, xenon hexafluoride is obtained (Xe + 3F2 -> XeF6). Heating XeF6 at 50 °C in a sealed quartz vessel gives the oxyfluoride XeOF4, since the quartz (silica) vessel itself is slowly attacked (2XeF6 + SiO2 -> 2XeOF4 + SiF4); further reaction under similar conditions progressively replaces more fluorine with oxygen, eventually giving xenon trioxide (2XeOF4 + SiO2 -> 2XeO2F2 + SiF4; 2XeO2F2 + SiO2 -> 2XeO3 + SiF4). On complete hydrolysis with water vapour, XeF6 similarly gives xenon trioxide directly (XeF6 + 3H2O -> XeO3 + 6HF); XeO3 is a dangerously explosive, colourless solid. When XeF6 is treated with 2.5 M sodium hydroxide, sodium perxenate is obtained (2XeF6 + 16NaOH -> Na4XeO6 + Xe + O2 + 12NaF + 8H2O), a compound whose perxenate ion (XeO6^4-) is a strong oxidising agent — for instance, it is able to oxidise manganese(II) ion all the way to permanganate even without a catalyst (5XeO6^4- + 2Mn2+ + 14H+ -> 2MnO4- + 5XeO3 + 7H2O). Xenon also reacts directly with platinum hexafluoride, PtF6, to give an orange-yellow solid formulated [XePtF6] (insoluble in carbon tetrachloride) — historically the very first noble-gas compound ever prepared (by Neil Bartlett in 1962), which inspired the subsequent discovery of the xenon fluorides. Xenon difluoride and xenon hexafluoride can also act as fluoride-ion acceptors, forming addition/adduct compounds with strong fluoride acceptors such as antimony pentafluoride and tantalum pentafluoride (XeF2.2SbF5, XeF2.2TaF5) and with boron trifluoride or alkali-metal fluorides (XeF6.BF3, XeF6.MF where M is an alkali metal); there is also some evidence for the existence of xenon dichloride, XeCl2, though it is far less stable than the fluorides. Krypton, being smaller and holding its electrons more tightly than xenon, is far less reactive, but does form krypton difluoride, KrF2, when an electric discharge is passed through krypton and fluorine at about -183 °C, or when the gases are irradiated in the presence of antimony pentafluoride, giving an adduct KrF2.2SbF5.

Structures of the xenon compounds (by VSEPR theory): xenon difluoride (XeF2) is sp3d hybridised and linear; xenon tetrafluoride (XeF4) is sp3d2 hybridised and square planar; xenon hexafluoride (XeF6) is sp3d3 hybridised and adopts a distorted octahedral shape (owing to the stereochemically active lone pair); the oxyfluoride XeOF2 is sp3d hybridised and T-shaped; XeOF4 is sp3d2 hybridised and square pyramidal; and xenon trioxide (XeO3) is sp3 hybridised and pyramidal. …