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.
• Longest lived isotope is 222Rn (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
! In XeF6 and XeF82–, 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:
Xe + F2
XeFÄ2Hfo = –105 kJ
• XeF2 is a powerful oxidant:
XeF2 + 2H+ + 2e– ÷ Xe + 2HF
Eo = +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
XeF82–.
! 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).
• KrF2 is unstable: Kr(g) + F2(g) ÷ KrF2(g) ÄHfo = +63 kJ
• Solid has two structures, both tetragonal: low temperature á
(<< –80 oC; isomorphous with RT XeF2) and higher
temperature â.1,2
á-KrF2 at –125 oC
â-KrF2 at –80 oC
1
J. F. Lehman, D. A. Dixon, G. J. Schrobligen, Inorg. Chem., 2001, 40, 3002.
2
R. 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
disproportionates to the perxenate ion, XeO64–:
XeO3 + OH– º HXeO4–
K = 1.5 x 10–3
2HXeO4– + 2OH– ÷ XeO64– + Xe + O2 + 2H2O
• XeO64– is isoelectronic and isostructural with paraperiodate.
! Both XeO3 and XeO64– are powerful oxidants:
XeO3 + 6H+ + 6e– º Xe + 3H2O
XeO64– + 6H+ + 2e– º XeO3 + 3H2O
Eo = +2.12 V
Eo = +2.36 V