354 THE NOBLE GASES
- The increase in melting point and boiling point, and the very
narrow liquid range.
- The large ionisation energies, as expected for atoms with com
plete quantum levels.
- The small negative electron affinities of helium and neon.
The increases in melting point and boiling point arise because of
increased attraction between the/ree atoms; these forces of attraction
are van der Waal's forces (p. 47) and they increase with increase
of size. These forces are at their weakest between helium atoms, and
helium approaches most closely to the 'ideal gas'; liquid helium
has some notable characteristics, for example it expands on cooling
and has very high thermal conductivity.
CHEMICAL PROPERTIES (1)
The simple fact that the noble gases exist as separate atoms—a
uniqiie property at ordinary temperatures—is sufficient indication
of their chemical inactivity. Calculations of the heats of formation
of hypothetical noble gas ionic compounds have been made, using
methods similar to those described in Chapter 3 for kNaC! 3 ' or
%MgCl'; they indicated that, if the noble gases are to form cations
X+, then the anion must have a large electron affinity to "compensate"
for the large ionisation energy of X (Table 12.1). The discovery by
Bartlett that the compound platinum(VI) fluoride, PtF 6 , had a
sufficiently large electron affinity to unite directly with molecular
oxygen O 2 (first ionisation energy 1176 kJ mol ) to form the
essentially ionic compound O 2 PtF 6 (i.e. O2[PtF 6 ]~), suggested
that xenon (1st ionisation energy 1169 kJ moP^1 ) might form a
similar compound XePtF 6 , and this compound was made by direct
reaction of xenon with platinum(VI) fluoride. The further chemistry
of the noble gases is described later.
OCCURRENCE AND ISOLATION
The most important source of helium is the natural gas from
certain petroleum wells in the United States and Canada. This gas
may contain as much as 8 % of helium. Because helium has a lower
boiling point (Table 12.1) than any other gas, it is readily obtained
by cooling natural gas to a temperature at which all the other gases
are liquid (77 K); almost pure helium can then be pumped off. The
yearly production in this way may be many millions of m^3 of gas,
but something like 1011 m^3 per year is still wasted.
