Grup 18 - The Elements! All fund in minute quantities in air. Argn is mst abundant (and cheapest), cmprising 0.934% f air by vlume.! Althugh rare n earth, helium is the secnd mst abundant element in the universe (H, 76%; He, 23%), being a majr cmpnent f stars.! All radn istpes are shrt-lived and are cntinuusly being prduced by natural decay prcesses. 222 Lngest lived istpe is Rn (á, t = 3.825 days).! Cndensed phases are held tgether by van der Waals frces, which increase smthly dwn the grup. Element b.p. (K) I (kj/ml) He 4.18 2372 Ne 27.13 2080 Ar 87.29 1520 Kr 120.26 1351 Xe 166.06 1169 Rn 208.16 1037 ½
Helium! Helium is fund in certain natural gas depsits (e.g., in Kansas), where it prbably frms as a result f radiactive decay in the rcks.! Because f its lw biling pint, it is used widely in crygenic applicatins. Als used in airships (e.g., Gdyear blimp) and as a balln filler. Used as a fill gas fr deep diving, because it is less sluble in bld than nitrgen, thus aviding the "bends." 4! The He istpe has the lwest knw biling pint, 4.12 K, at which pint it is called He-I. On cling t 2.178 K a phase transitin t He-II ccurs. He-II has an expanded vlume, almst n viscsity, and is supercnducting. He-II can readily flw uphill t equalize vlumes in adjacent vessels. It cannt be stred in glass Dewars, because it leaks thrugh glass int the evacuated space, psing an explsin risk.! At 0.9 K, He flats n He, permitting separatin f the 3 4 istpes.
Chemistry f the Nble Gases! Befre 1962 it was assumed that n cmpunds culd be frmed, wing t the "stability f the ctet."! In 1962 Neil Bartlett reacted O 2 with PtF 6, frming + [O 2] [PtF 6]. O 2 and Xe have similar inizatin energies, s Bartlett tried the same reacin with Xe, prducing a cmpund reprted + t be [Xe] [PtF 6]. Tday the reacin is believed t be the fllwing: + + Xe + 2PtF 6 [XeF] [PtF 6] + PtF 5 [XeF] [Pt2F 11] Shrtly after this discvery, the xenn flurides were prepared by direct cmbinatin at high temperature and pressure. Xe + xsf 2 XeF 2, XeF 4, XeF6 All are slids.! Tday many cmpunds and ins f Xe with F and O are knwn, and even sme with Kr and F are knwn, althugh Kr cmpunds are stable nly at very lw temperatures.
Structures! Mst nble gas cmpunds have structures predictable by VSEPR cnsideratins. D h D 4h C 3v C 4v 2! In XeF 6 and XeF 8, the lne pairs d nt seem t ccupy a fixed psitin, and the structures are fluxinal, n average distrted ctahedral (C 3v) and distrted square antiprism (~D ), respectively. 4d
Xenn Flurides! XeF 2 frms with alacrity when a stichimetric mixture f Xe and F in a glass flask is irradiated with sunlight: 2 Xe + F XeF H = 105 kj 2 Ä 2 f XeF 2 is a pwerful xidant: XeF + 2H + 2e Xe + 2HF 2 + E = +2.64 V! Nnetheless, bth XeF 2 and XeF 4 are nly mildly reactive, whereas XeF 6 attacks glass: 2XeF + SiO 2XeOF + SiF 6 2 4 4! The lne pair n XeF 6 allws it t act as a Lewis base, frming adducts such as XeF 6:AsF 3, XeF 6:BF 3, XeF 6:SbF 5. XeF 6 can als act as a Lewis acid in frming XeF 7 and 2 XeF. 8! Xenn flurides hydrlyze t give characteristic mixtures f prducts: 2XeF 2 + 2H2O 2Xe + O 2 + 4HF (slw) 4XeF 4 + 8H2O 2Xe + O 2 +16HF + 2XeO 3 (vilent!) XeF + 3H O 6HF + XeO (vilent!) 6 2 3! Carefully cntrlled hydrlysis f XeF 6 with the stichimetric amunt f H2O gives XeOF 4, a clrless liquid. XeF + H O XeOF + 2HF 6 2 4! Xenn flurides have been used as flurinating agents. 4SF 5 + XeF 4 4SF 6 + Xe 2C H + XeF 2C H F + Xe + H 6 6 2 6 5 2
Kryptn Fluride! Kr has ne mlecular fluride, KrF 2, which is frmed by direct cmbinatin at lw temperature, and cmpunds with + + the ins KrF and Kr2F 3 and [MF 6] anins (M = As, Sb, Bi). KrF 2 is unstable: Kr(g) + F 2(g) KrF 2(g) ÄH f = +63 kj Slid has tw structures, bth tetragnal: lw temperature á (<< 80 C; ismrphus with RT XeF 2) and higher temperature â. 1,2 á-krf at 125 C 2 2 â-krf at 80 C 1 J. F. Lehman, D. A. Dixn, G. J. Schrbligen, Inrg. Chem., 2001, 40, 3002. 2 R. D. Burbank, W. E. Falcner, W. A. Sunder, Science, 1972, 178, 1285.
XeO 3 and Its Anins! Slutins f XeO 3, frmed by hydrlysis f XeF 4 r XeF 6, are stable up t 11 M. With careful evapratin XeO 3 is btained as a white, deliquescent slid. XeO is dangerusly explsive! 3! XeO 3 disslves in water as mlecules, but in base an equilibrium with OH ccurs, frming HXeO 4, which slwly 4 disprprtinates t the perxenate in, XeO 6 : 3 XeO 3 + OH HXeO4 K = 1.5 x 10 4 2HXeO 4 + 2OH XeO 6 + Xe + O 2 + 2H2O 4 XeO is iselectrnic and isstructural with paraperidate. 6! Bth XeO and XeO are pwerful xidants: XeO + 6H + 6e Xe + 3H O E = +2.12 V XeO + 6H + 2e XeO + 3H O E = +2.36 V 4 3 6 + 3 2 4 + 6 3 2