Intermolecular Forces

Intermolecular forces are the electrical forces of attraction (and, at very short range, of repulsion) that act between neighbouring molecules of a substance -- they are distinct from the much stronger intramolecular forces (covalent/ionic bonds) that hold the atoms of a single molecule together. Because these forces are fundamentally attractive at normal separations and weaken rapidly as the distance between molecules increases, their overall strength is what decides which of the three physical states -- solid, liquid or gas -- a given substance adopts under a given set of conditions: intermolecular attraction is strong in solids, weaker (but still significant) in liquids, and very weak in gases. Many everyday physical properties of a substance -- its melting point, boiling point, vapour pressure, viscosity, rate of evaporation, surface tension and solubility -- are all, at heart, a reflection of how strong the attractive forces between its molecules are. When a solid melts, the intermolecular forces holding its particles in a fixed lattice are only partly overcome, so the particles gain some freedom t …