Interatomic and Intermolecular Forces

Ordinary matter is an aggregate of atoms or of molecules. A molecule, in turn, may be defined as a grouping of a limited number of atoms which are strongly bonded together but whose bonds with the atoms of other adjacent molecules are relatively weak. These groups of atoms therefore act as a unit in the constitution of matter. Forces of attraction or bonds which are responsible for the aggregation of atoms and molecules into ordinary matter can be divided into primary bonds or chemical bonds, and secondary bonds or intermolecular attraction forces, frequently called van der Waals forces. Primary bonds are of three general types: (1) ionic or electrostatic bonds, (2) covalent, atomic or homopolar bonds, and (3) metallic bonds. This classification of primary bonds is, however, not a rigorous one. Thus, there are many bonds which do not strictly belong to any of the three types but are intermediate between truly ionic and truly covalent bonds, between ionic and metallic bonds, or between covalent and metallic bonds.