London force, dispersion force
The weakest of the imtermolecular forces. Present
on all particles and increasing strength with increasing
size. Results from the fact that a preponderance
of electrons can end up on one side of an atom.
The dispersion force which in fact is an induced
dipole - induced dipole interaction depends on the
polarisability of the interacting molecules and
is inversely proportional to the sixth power of
separation. In the case of e.g. two CH4 molecules
at a separation of 3Å, the dispersion interaction
energy is of the order of -1.1 Kcal/mole.