Dispersion Forces or London Forces

Why should non-polar atoms attract each other at all?

An isolated atom or a non-polar molecule has its electron cloud distributed symmetrically, so on average it carries no dipole moment. Yet even such species can develop a fleeting dipole. Here is the mechanism.

Imagine two atoms, A and B, sitting close together (see Fig. 5.1). At any instant, the electron cloud around atom A can momentarily become lopsided — more charge density on one side than the other — purely by chance, as electrons are always in motion. This creates a short-lived instantaneous dipole on A. That instantaneous dipole distorts the electron cloud of the neighbouring atom B, pushing its electrons away (or drawing them in) and thereby inducing a matching dipole on B.

The two temporary, oppositely-oriented dipoles on A and B then attract each other. The same picture applies to non-polar molecules, not just atoms.

London (dispersion) forces

This attraction between two transient, induced dipoles is named after the German physicist Fritz London, who first proposed it — hence London forces, also called dispersion forces. Their key features:

• They are always attractive.
• Interaction energy falls off very steeply with distance: it is proportional to $\dfrac{1}{r^{6}}$, where $r$ is the separation between the two particles.
• Because of that steep fall-off, they only matter at short range (roughly up to ~500 pm).
• Their strength grows with the polarisability of the particle — how easily its electron cloud can be distorted. …