a) Crystalline solids : Crystalline solids have long range order. The long range order means the atoms or ions or molecules are arranged in a regular fashion and this symmetrical arrangement extends throughout the crystal length.

b) Amorphous solids : An amorphous solid differs from a crystalline substance in being without any shape of its own and has a completely random particle arrangements, i.e. no regular arrangement. Example : Glass, Piastic.