Methane has no such charges, and hence has little tendency for two
molecules to stick together. The only attractions between molecules
are the weak van der Waals forces mentioned previously for H2
and He. These attractions arise because, although an atom in a molecule
may be electrically nonpolar on the average over a finite period of
time, at any given instant the electrons may not be distributed symmetrically
around the nucleus. This is illustrated for three atoms at the right.
The first drawing shows the time average, with a symmetrical distribution
of electrons around each nucleus. The following three drawings show
"snapshots" of the atoms at three instants in time when the random
motion of electrons has brought about shortlived attractions between
atoms A and B, B and C, and A and C.These attractions may seem small,
but they are not insignificant. They are the forces between atoms
in neighboring methane molecules that make methane gas finally condense
to a liquid at -164C. The strengths of van der Waals forces depend
mainly on surface areas of molecules. Hence gaseous H2
molecules, which are smaller than CH4
molecules, must be cooled to -253C before they move slowly enough
that van der Waals attractions can make them stick to one another
in a liquid.