Dependence on Number of Electrons
IMF forces are known to be dependent on the instantaneous dipole moment of a molecule. Although the molecules that experience Van der Waals forces as their principle IMF's are nonpolar molecules, the electrons in these types of molecules are not in fixed locations. For this reason, at any given point in time the molecules will undergo an instantaneous dipole, or will become polar at that particular instant. There is a high probability that the electrons will not be perfectly balanced on each side of the molecule during this dipole moment--if they are not balanced, then either a positive or negative charge on the molecule will be momentarily established. Thus at that instant in time, molecules of the same kind will attract or repel each other depending on their charge (even though the molecules are still nonpolar).