Intermolecular forces are either the forces of attraction or forces of repulsion between interacting molecules. When two molecules come very close to each other, the repulsion between the electron clouds and that between their nuclei comes into play. The magnitude of the repulsion rises very rapidly as the distance separating the molecules decreases. This type of force is the repulsive intermolecular force. This repulsive force limits the compression level of liquids and solids.

Attractive intermolecular forces are known as van der Waals forces. Force example, molecules of liquid water is attracted to each other by electrostatic forces that are known as van der Waals forces or van der Waals bonds. The charge distribution in water molecule is not symmetrical and leads to a microscopic separation of the positive and negative charge centers. That is, the hydrogen will be slightly positive with respect to the oxygen, which has high elecronegativity even though the molecule as a whole is still electrically neutral. This charge separation is the cause of van der Waals force.

Non-polar molecules also experience some van der Waals bonding. Such molecules do not have permanent electric poles but they can have instantaneous electric poles which change or oscillate with time. These fluctuating molecular polarities lead to a net attraction force between molecules which allow non-polar substances to form liquids. The electric field from one instantaneous electric pole tends to polarize a neighboring molecule so that it will be attracted.

Thermal energy is the energy of a body arising from motion of its atoms or molecules. It is directly proportional to the temperature of the substance. This movement of particles is called thermal motion. The force due to thermal energy of molecules acts opposite to the attractive intermolecular force, and it is a dispersive force.