18.12 Occurrence, preparation, and properties of the noble gases  (Page 2/4)

Argon was useful in the manufacture of gas-filled electric light bulbs, where its lower heat conductivity and chemical inertness made it preferable to nitrogen for inhibiting the vaporization of the tungsten filament and prolonging the life of the bulb. Fluorescent tubes commonly contain a mixture of argon and mercury vapor. Argon is the third most abundant gas in dry air.

Krypton-xenon flash tubes are used to take high-speed photographs. An electric discharge through such a tube gives a very intense light that lasts only $\frac{1}{\mathrm{50,000}}$ of a second. Krypton forms a difluoride, KrF ₂ , which is thermally unstable at room temperature.

Stable compounds of xenon form when xenon reacts with fluorine. Xenon difluoride, XeF ₂ , forms after heating an excess of xenon gas with fluorine gas and then cooling. The material forms colorless crystals, which are stable at room temperature in a dry atmosphere. Xenon tetrafluoride, XeF ₄ , and xenon hexafluoride, XeF ₆ , are prepared in an analogous manner, with a stoichiometric amount of fluorine and an excess of fluorine, respectively. Compounds with oxygen are prepared by replacing fluorine atoms in the xenon fluorides with oxygen.

When XeF ₆ reacts with water, a solution of XeO ₃ results and the xenon remains in the 6+-oxidation state:

Dry, solid xenon trioxide, XeO ₃ , is extremely explosive—it will spontaneously detonate. Both XeF ₆ and XeO ₃ disproportionate in basic solution, producing xenon, oxygen, and salts of the perxenate ion, ${\text{XeO}}_6{}^{\mathrm{4-}},$ in which xenon reaches its maximum oxidation sate of 8+.

Radon apparently forms RnF ₂ —evidence of this compound comes from radiochemical tracer techniques.

Unstable compounds of argon form at low temperatures, but stable compounds of helium and neon are not known.

Key concepts and summary

The most significant property of the noble gases (group 18) is their inactivity. They occur in low concentrations in the atmosphere. They find uses as inert atmospheres, neon signs, and as coolants. The three heaviest noble gases react with fluorine to form fluorides. The xenon fluorides are the best characterized as the starting materials for a few other noble gas compounds.

Chemistry end of chapter exercises