Crystal Structure of Solid Materials:

A crystal is a solid whose constituents are arranged in a systematic manner. The distinctive features of solids are their definite shape, their hardness and their fixed volume.

These characteristics can be explained on the basis of following facts:

(1) The constituents units (atoms, ions or molecules) of solids are held very close to each other so that the packing of the constituents is very efficient, consequently solids have high densities.

(2) Since the constituents of the solids are closely packed, it imparts rigidity and hardness to the solids.

(3) The constituents of solids are held together by strong forces of attraction. This results in their having definite shape and fixed volume.

Information regarding the above mentioned characteristics in solids can be obtained by the study of crystal structure, i.e., internal arrangement of atoms, ions or molecules in space. We may consider solids to exist in two different states, namely, amorphous and crystalline.

An amorphous solid is a substance whose constituents do not possess an orderly arrangement. The size of ordered region in these substances is limited to a few molecule distances. Amorphous solids are also called non-crystalline solids. A crystalline solid is a substance whose constituents possess an orderly arrangement in a definite geometric pattern.

A crystalline solid can be either a single crystal, where the solid consists of only one crystal or an aggregate of many small crystals or grains which are highly ordered crystalline regions of irregular size and orientation. The second type of crystalline materials is known as polycrystalline materials. Single crystals have long range order.