London forces, named after the German physicist Fritz London, are weak intermolecular forces that arise from the attractive force between transient dipoles in otherwise nonpolar molecules. London forces are also called London dispersion forces.
London forces can be exhibited by nonpolar species because electrons move about a molecule probabilistically. There is a high chance that the electron density will not be evenly distributed throughout a nonpolar molecule. When an uneven distribution occurs, a temporary dipole is created. This dipole may interact with other nearby dipoles.
Electron density in a molecule may be redistributed by proximity to another dipole. Electrons will gather on the side of a molecule that faces a positive charge and retreat from a negative charge. Hence, a transient dipole can be produced by a nearby polar molecule, or even a transient dipole in another nonpolar molecule.
London forces are much weaker than other intermolecular forces such as ionic interactions, hydrogen bonding, or dipole-dipole interactions.
This phenomenon is the only intermolecular force present between nonpolar species such as helium, nitrogen, or methane (to name a few). Without London forces, there would be no attractive force between these molecules and they could not then be obtained in a liquid form.
London forces become stronger as the atom (or molecule) in question becomes larger. This is due to the increased polarizability of molecules with larger, more dispersed electron clouds. This trend is exemplified by the halogens (from smallest to largest: F2, Cl2, Br2, I2). Fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid.