An attractive force between atoms or molecules, named after J. D. van der Waals (1837–1923). The force accounts for the term a/V² in the van der Waals equation (see equation of state). These forces are much weaker than those arising from valence bonds and are inversely proportional to the seventh power of the distance between the atoms or molecules. They are the forces responsible for nonideal behaviour of gases and for the lattice energy of molecular crystals. There are three factors causing such forces: (1) dipole–dipole interaction, i.e. electrostatic attractions between two molecules with permanent dipole moments; (2) dipole-induced-dipole interactions, in which the dipole of one molecule polarizes a neighbouring molecule; (3) dispersion forces arising because of small instantaneous dipoles in atoms. A quantitative theory of van der Waals’ force was given by Fritz London (1900–54) in 1930 in terms of quantum mechanics. If the distance between molecules is greater than about 10 nanometres, then relativistic effects mean that the variation of the force is changed from the inverse of the seventh power to the inverse of the eighth power. This has been observed for colloids.

It is common to express van der Waals’ force in terms of the energy of interaction between molecules, with this energy being inversely proportional to the sixth power of the distance between the molecules in the nonrelativistic case and inversely proportional to the seventh power in the relativistic case.