Explain formation of London forces.

Explanation

Atoms and nonpolar molecules are electrically symmetrical and have no dipole moment because their electronic charge cloud is symmetrically distributed.
But a dipole may develop momentarily even in such atoms and molecules.
Suppose we have two atoms .A. and .B. in the close vicinity of each other.

It may so happen that momentarily electronic charge distribution in one of the atoms .A., becomes unsymmetrical i.e., the charge cloud is more on one side than the other.

This results in the development of instantaneous dipole on the atom .A. for a very short time. This instantaneous or transient dipole distorts the electron density of the other atom .B., which is close to it and as a consequence a dipole is induced in the atom .B..
The temporary dipoles of atom .A. and .B. attract each other. Similarly temporary dipoles are induced in molecules also.