The most common type of intermolecular force,found in all molecular substance,is referred to as a dispersion forces.It is basically electrical in nature involving an attraction between temporary or induced dipoles in adjacent molecules.
In 1930 Fritz London found he could account for a weak attraction between any two molecules.Let's use neon to illustrate .The electron of this atom move about the nucleus so that over a period of time they have a spherical distribution.The electrons spend as much time on one side of the nucleus as on the other.Experimentally you find that the atom has no dipole moment.However,at the any instant,more electrons may be on one side of the nucleus than on the other.This makes atom a small instantaneous dipole,with one side having a partial negative charge and the other side a partial positive charge.
Now imagine that another neon atom is close to this first atom lets say next to partial negative charge that appeared at that instant.This partial negative charge repels the electrons in the second atom, so at this instant it too becomes a small dipole.As these two dipoles are oriented with the partial negative charge on one atom next to the partial positive charge of other atom,it creates an attractive force between the two atoms.
Electrons in constant motion affects the motion of electrons on other atom.Due to which,the instantaneous dipoles of the atoms tend to change togather,maintaining the net attractive force.Such instantaneous changes of electron distributions occur in all molecules.For this reason,an attractive force always exists between any two molecules.
Thus, we can see that we define the Dispersion forces or London forces as the weak attractive forces between molecules resulting from the small,instantaneous dipoles that occur because of the varying positiions of the electrons during their motion about nuceli.
All molecule have dispersion forces.The strength of these forces depends on two factors:-
- the number of electrons in the atoms that make up the molecule.
- the ease with which electrons are dispersed to form temporary dipoles.
Both these factors increases with the increasing molecular size.large size molecules are made up of more and or large atoms.The outer electrons in larger atoms are relatively far from the nucleus and are easier to disperse than the electrons in small atoms.In general,molecular size and molar mass parallel one another.Thus,within a given class of substances we can say that as molar mass increases,dispersion forces become stronger and the boiling point of non polar molecular substances increases.