Covalent bonding in molecules such as H₂, N₂ or CH₄ is characterized by a nearly "complete" sharing of valence electrons between adjacent nuclei and even the nuclei beyond the adjacent ones, as in the case of butadiene and benzene. This is a predominant form of bonding which really gives the "structures" to molecules. There are other forms of "bonding" in which features that are complementary to covalent bonding dominate in the description of the bonding process.

In ionic bonding (such as in the NaCl lattice), the ions are held together in periodic arrays and the major attractive force is the electrostatic attraction between the ions. In the repulsive force between the ions, in addition to the electrostatic repulsion between the ions, there is a repulsion between the electronic clouds of the ions. Hydrogen bonding is another common bonding process wherein a hydrogen atom is normally shared between two electronegative atoms. The distances for hydrogen bonds are in the range of 1.5 to 2.5

while the distances in covalent bonds are in the range of 0.7 to 2

. The distances between adjacent ions in an ionic crystal are generally determined by the sizes of ions and can vary between 1 and 5

At low temperatures even "nonreactive" species such as rare gas atoms condense to form liquids and solids and this is due to the existence of intermolecular forces. Solids formed by Ar, CH₄ or benzene are referred to as molecular solids. In these solids, the molecularity of the constituent species is intact and the molecules are held together by weak intermolecular forces. The strength of intermolecular forces is typically 0 to 3 kcal/mol, the strength of hydrogen bonds is between 3 to 8 kcal/mol while the strength of ionic and covalent bonding is in the range of 25 to 250 kcal/mol. In the present and the next lecture, we shall study the nature and consequences of intermolecular forces.

Among the intermolecular forces the most universal one is the dispersion force. The dispersion force exists between all molecules even when they do not possess any charge or dipole moment. It arises from a correlated motion of electrons between two molecular entities. Although the average dipole moment (defined as the charge separation multipied by the distance separating the charges; this will be elaborated in the next section) of a neutral molecule is zero, the instantaneous dipole moment (the vector sum of the dipole moments due to the individual electrons, with the nuclei taken as the origin) is not zero. The instantaneous dipole moment of a molecule can influence the polarization cloud of a neighbouring molecule and be influenced by the instantaneous dipole moment of the adjacent molecule. This is the mechanism of dispersion interaction.