Illustrated Glossary of Organic Chemistry

London force (dispersion force; London dispersion force): A noncovalent molecular force caused by attraction of polarized electron clouds. The electron cloud polarization is induced: it is caused when the electron clouds repel each another, creating adjacent regions of electron deficiency (δ⁺) and electron excess (δ^-). It has also been suggested the polarizations result from random fluctuations in electron density.

<--- Far apart --->

The electron clouds of two atoms in close proximity cause mutual polarization, resulting in London forces.