Origin of London forces
In order to understand the origin of London forces, let us consider a monoatomic molecule of neon. Each neon atom has its electron cloud symmetrically distributed around the nucleus. It can be imagined that due to rapid movement of electrons, the electron distribution of the atom may become unsymmetrical with slight increase of electron density on one side. This would lead to the separation of positive and negative ends at a particular instant. In other words, the molecule behaves like a temporary dipole. Such an atom having a small temporary dipole is called instantaneous dipole. This instantaneous dipole influences the electron distribution of other molecules in its close vicinity and induces dipole in them also. The attractive forces between the instantaneous dipoles and induced dipoles are called van der Waal's forces.
It may be noted that London forces are quite weak. The strength of London forces lies below 12.5 kJ mol-1. The magnitude of these forces determines the melting and boiling points of various substances. Although these forces are weak, yet their magnitude increases with the increase in:
(i) number of electrons in the molecule(ii) molecular mass of the substance and
(iii) surface area of the molecule.For e.g., among halogens the number of electrons in a molecule as well as the molecular mass increases from fluorine to iodine. Consequently, the magnitude of London forces increases, thereby increasing the melting and boiling points from fluorine to iodine. Now if boiling points of n-butane and iso-butane are compared it is found that n-butane has higher boiling point. This is due to the fact that molecules of n-butane have greater surface area and hence have stronger London forces.
From the above discussion it follows that molecular solids usually have low melting and boiling points due to very weak inter-particle forces.
