Group 18 - The Elements ! All found in minute quantities in air. • Argon is most abundant (and cheapest), comprising 0.934% of air by volume. ! Although rare on earth, helium is the second most abundant element in the universe (H, 76%; He, 23%), being a major component of stars. ! All radon isotopes are short-lived and are continuously being produced by natural decay processes. 222 • Longest lived isotope is Rn (á, t½ = 3.825 days). ! Condensed phases are held together by van der Waals forces, which increase smoothly down the group. Element b.p. (K) I (kJ/mol) He 4.18 2372 Ne 27.13 2080 Ar 87.29 1520 Kr 120.26 1351 Xe 166.06 1169 Rn 208.16 1037 Helium ! Helium is found in certain natural gas deposits (e.g., in Kansas), where it probably forms as a result of radioactive decay in the rocks. ! Because of its low boiling point, it is used widely in cryogenic applications. • Also used in airships (e.g., Goodyear blimp) and as a balloon filler. • Used as a fill gas for deep diving, because it is less soluble in blood than nitrogen, thus avoiding the "bends." ! The 4He isotope has the lowest know boiling point, 4.12 K, at which point it is called He-I. • On cooling to 2.178 K a phase transition to He-II occurs. • He-II has an expanded volume, almost no viscosity, and is superconducting. • He-II can readily flow uphill to equalize volumes in adjacent vessels. • It cannot be stored in glass Dewars, because it leaks through glass into the evacuated space, posing an explosion risk. ! At 0.9 K, 3He floats on 4He, permitting separation of the isotopes. Chemistry of the Noble Gases ! Before 1962 it was assumed that no compounds could be formed, owing to the "stability of the octet." ! In 1962 Neil Bartlett reacted O2 with PtF6, forming + – [O2] [PtF6] . • O2 and Xe have similar ionization energies, so Bartlett tried the same reacion with Xe, producing a compound reported + – to be [Xe] [PtF6] . • Today the reacion is believed to be the following: ÷ + – ÷ + – Xe + 2PtF6 [XeF] [PtF6] + PtF5 [XeF] [Pt2F11] • Shortly after this discovery, the xenon fluorides were prepared by direct combination at high temperature and pressure. Xe + xsF2 XeF2, XeF4, XeF6 • All are solids. ! Today many compounds and ions of Xe with F and O are known, and even some with Kr and F are known, although Kr compounds are stable only at very low temperatures. Structures ! Most noble gas compounds have structures predictable by VSEPR considerations. D4h D4h C3v C4v 2– ! In XeF6 and XeF8 , the lone pairs do not seem to occupy a fixed position, and the structures are fluxional, on average distorted octahedral (C3v) and distorted square antiprism (~D4d), respectively. Xenon Fluorides ! XeF2 forms with alacrity when a stoichiometric mixture of Xe and F2 in a glass flask is irradiated with sunlight: o Xe + F2 XeFÄ2Hf = –105 kJ • XeF2 is a powerful oxidant: + – ÷ o XeF2 + 2H + 2e Xe + 2HF E = +2.64 V ! Nonetheless, both XeF2 and XeF4 are only mildly reactive, whereas XeF6 attacks glass: ÷ 2XeF6 + SiO2 2XeOF4 + SiF4 ! The lone pair on XeF6 allows it to act as a Lewis base, forming adducts such as XeF6:AsF3, XeF6:BF3, XeF6:SbF5. – • XeF6 can also act as a Lewis acid in forming XeF7 and 2– XeF8 . ! Xenon fluorides hydrolyze to give characteristic mixtures of products: ÷ 2XeF2 + 2H2O 2Xe + O2 + 4HF (slow) ÷ 4XeF4 + 8H2O 2Xe + O2 +16HF + 2XeO3 (violent!) ÷ XeF6 + 3H2O 6HF + XeO3 (violent!) ! Carefully controlled hydrolysis of XeF6 with the stoichiometric amount of H2O gives XeOF4, a colorless liquid. ÷ XeF6 + H2O XeOF4 + 2HF ! Xenon fluorides have been used as fluorinating agents. ÷ 4SF5 + XeF4 4SF6 + Xe ÷ 2C6H6 + XeF2 2C6H5F + Xe + H2 Krypton Fluoride ! Kr has one molecular fluoride, KrF2, which is formed by direct combination at low temperature, and compounds with + + – the ions KrF and Kr2F3 and [MF6] anions (M = As, Sb, Bi). ÷ o • KrF2 is unstable: Kr(g) + F2(g) KrF2(g) ÄHf = +63 kJ • Solid has two structures, both tetragonal: low temperature á o (<< –80 C; isomorphous with RT XeF2) and higher temperature â.1,2 o o á-KrF2 at –125 C â-KrF2 at –80 C 1J. F. Lehman, D. A. Dixon, G. J. Schrobligen, Inorg. Chem., 2001, 40, 3002. 2R. D. Burbank, W. E. Falconer, W. A. Sunder, Science, 1972, 178, 1285. XeO3 and Its Anions ! Solutions of XeO3, formed by hydrolysis of XeF4 or XeF6, are stable up to 11 M. • With careful evaporation XeO3 is obtained as a white, deliquescent solid. K XeO3 is dangerously explosive! ! XeO3 dissolves in water as molecules, but in base an – – equilibrium with OH occurs, forming HXeO4 , which slowly 4– disproportionates to the perxenate ion, XeO6 : – º – –3 XeO3 + OH HXeO4 K = 1.5 x 10 – – ÷ 4– 2HXeO4 + 2OH XeO6 + Xe + O2 + 2H2O 4– • XeO6 is isoelectronic and isostructural with paraperiodate. 4– ! Both XeO3 and XeO6 are powerful oxidants: + – º o XeO3 + 6H + 6e Xe + 3H2O E = +2.12 V 4– + – º o XeO6 + 6H + 2e XeO3 + 3H2O E = +2.36 V.