JEE Chemistry The P Block Elements Notes Set 17

Group 18: Noble Gases

1. The members are He, Ne, Ar, Kr, Xe and Rn with complete octets (He has a duplet). Argon is the most abundant noble gas in air.
2. Ionisation enthalpy falls He > Ne > Ar > Kr > Xe > Rn. Atomic radius, density, melting point and boiling point rise down the group. They exist as monatomic gases with only weak dispersion forces.
3. Helium has the lowest boiling point of any substance (about 4 K), and liquid He exists in two forms. It is only slightly soluble in blood, so a He-O2 mixture is used by divers to reduce the risk of bends. Helium and neon diffuse through glass, rubber and some metals.
4. Ar, Kr and Xe form clathrates because their atoms fit in cages of water or quinol; He and Ne are too small and so do not.
5. Xenon forms compounds with the most electronegative elements F and O. Xe reacts with F2 in a nickel vessel on heating: excess Xe gives \( \ce{XeF2} \), a 1 : 5 ratio gives \( \ce{XeF4} \) and a large excess of F2 (1 : 20, high pressure) gives \( \ce{XeF6} \).

1. Hydrolysis: \( \ce{6XeF4 + 12H2O -> 4Xe + 2XeO3 + 24HF + 3O2} \), \( \ce{XeF6 + 3H2O -> XeO3 + 6HF} \) and \( \ce{XeF6 + H2O -> XeOF4 + 2HF} \) (partial). XeF2 hydrolyses slowly to Xe, HF and O2.
2. Xenon fluorides are strong fluorinating and oxidising agents. XeF6 reacts with silica, so it cannot be kept in glass: \( \ce{2XeF6 + SiO2 -> 2XeOF4 + SiF4} \).
3. Fluoride ion acceptors and donors: XeF6 with a fluoride gives \( \ce{XeF7-} \) and \( \ce{XeF8^2-} \); XeF2 gives adducts with SbF5 such as \( \ce{[XeF]+[SbF6]-} \).
4. Xenon is also used in lamps, and He is used in weather balloons and for cooling superconducting magnets.