Here, an ‘array of atoms’ is arranged in a uniform interval to form a crystal structure. The definite shape of solids is determined by the crystalline structure. In solids, the atoms are arranged in three dimensional plane to form a lattice and each atom has a well defined equilibrium due to inter-atomic forces. Diamonds, ice, quartz are some of the solids which exhibit crystalline structure even in their external appearance. Most of the substances that occur in nature have crystalline form in their atomic arrangement. One of the common forms is the cubic crystalline form. Modifications in cubic form are: 1. Body centred 2. Face centred