The ionic, covalent and coordinate bond arises due to attractive forces between atoms. Vander Waal (Dutch physicist, 1873) was the first to propose the existence of attractive forces between the atoms of inert gases with fully filled orbitals. These forces also exist between non-polar molecules as well as polar molecules. The attractive interactions between the molecules are responsible for bringing the molecules close together. The attractive interactions between the different molecules of a substance are called intermolecular forces.
The magnitude of these forces is maximum in the solids and decreases on passing from solid to liquids and from liquid to gaseous state. Vander Waal successfully explained the liquefaction of gases on the basis of inter molecular forces. These forces are purely electrostatic and thus physical in nature.