Molecular solid

In molecular solids, the constituents units are molecules, which do not carry any charge. The attractive forces binding the molecules together are of two types:

Dipole-dipole interactions

These types of forces occur in molecular solids which have polar molecules. A polar molecule has separate centres of positive and negative charges. These forces arise due to electrostatic attraction of the other molecule and vice-versa (The positive end of one molecule attracts the negative end of the other)

Van der Waal's forces

All molecules (polar and non-polar) exert a weak attraction upon the other, due to the electrostatic attraction of the electrons of one molecule to the nuclei of the other. These forces are known as van der Waals forces. They occur in all kinds of molecular solids including non-polar molecules such as H₂, O₂, Cl₂, CH₄, etc. and are present in both solid and liquid states. It has been observed that even non-polar substances such as hydrogen, chlorine, noble gases etc. can be liquefied due to these forces. The existence of weak attractive forces among non-polar molecules in their liquid and solid states was first proposed by J.D. Van der Waal's.