In a non-polar molecule, electron density fluctuates over time. On average, the molecule is non-polar but at any one time, the electric density can be unsymmetrical in a molecule. This creates an instantaneous dipole in the molecule.

The dipole in one molecule can induce a dipole in a neighbouring non-polar molecule (The partial positive charge on one part of the molecule will attract the electron density on the other)

These two dipoles are necessarily arranged so that they interact favourably with one another. This favourable interaction is responsible for the London force of attraction between the two molecules.

London forces depend on:

Number of electrons in molecule: More electrons → Greater fluctuations in electron density → Larger instantaneous and induced dipole
- Can be demonstrated by increasing boiling temperatures of mono-atomic molecules moving down the noble gases group in the periodic table
Shape and size of molecule : More points of contact → Greater overall London force

London forces are always present, whether or not a molecule has a permanent dipole.