Question

Why do London dispersion forces occur?

Answer 1

Different forces of attraction exist by the virtue of their masses, charges, magnetic effects, sharing of electrons or polarity but London dispersion forces are very weak and do not occur due to any of these reasons. Instantaneous dipoles play a crucial role in the formation of London forces.

Complete Step By Step Answer:
London dispersion forces are extremely weak, temporary and short range forces that are extremely easy to break and can be formed even in nonpolar molecules.
The reason for the occurrence of such forces is the fluctuation in the electrons allowed around the nucleus of different atoms or groups. Polarity is developed in between atoms or groups only when there is an electronegativity difference i.e. there is an unsymmetrical sharing of electrons or the electrons are being pulled away in one particular direction.
Symmetrical molecules or groups contain no such electronegativity difference and their electron clouds are evenly distributed around their nucleus. But at some points, the shifting or disturbance of electron clouds distorts the symmetrical distribution and an instantaneous and temporary dipole is generated. This dipole is capable of inducing a dipole in adjacent nonpolar groups or atoms which gives rise to weak London forces of attraction.

Note:
The moment the fluctuation or distortion of electron clouds in the group ceases due to environmental factors, the polarity vanishes away and the induced dipole always gets removed. Therefore the London forces are temporary and short lived in nature.