Van der Waals forces

Summary Van der Waals forces are among the forces that attract atoms and molecules together. Their importance follows from two unique properties. Firstly, they are universal. All atoms and molecules attract one another through this mechanism, so they account for phenomena, such as the cohesion of the inert gases in the solid and liquid states, and physical adsorption of molecules to solid surfaces, where no normal chemical bonds are formed. Secondly, the force is still significant when the molecules are comparatively far apart, and it is additive for large numbers of molecules. Van der Waals forces affect various properties of gases, and also give rise to an attractive force between two solid objects separated by a small gap, which is important in adhesion and in the stability of colloids. When the molecules are some distance apart the theoretical expressions for the force are particularly simple, and have now been verified experimentally, both for two isolated molecules and for two solid objects separated by a small gap.