Classsification of solids

Solids are classified into categories:

• Amorphous solids
• Crystalline solids

The two types of solids have different characteristics.

• Amorphous Solids. An amporphous solidd is a substance whose constituents do not possess an orderly arangement. Important examples of amorphous solids are glass and plastics. Although amorphous solids consist of microcrystalline substance but the orderly arrangement is restricted to very short distances. These distances are of the same order of magnitude as the interatomic distances.
• Crystalline Solids. A crystalline solid is a substance whose constituents possess an orderly arrangement in a definite geometric pattern. Some very common examples of crystalline substances are sodium chloride, sugar and diamond. The main characteristica of crystalline substances are:
1. Orderly arrangement. The costituent units of crystalline solids are arranged in an orderly fashion which repeats itself over very long distances as compared to interatomic distances. The arrangement of bricks in a wall can be considered as an example.  The arrangement is so well defined that the entire pattern can be repeated provided the arrangement of a few atoms is known.
2. Crystals are always bounded by plane faces.
3. The faces of crystals always meet at some fixed angles. 
   For any particular substances the angle between corresponding pair of faces is alway the same in all crystals.
   Different crystals of the same substance may sometimes appear to be different from outside, ( either due to different rate of growth by different faces or due to some damage to the corners or edges) but the interfacial angle is always the same. For example, sodium chloride grows from water solution as cubes but from 15% aqueous urea as an octahedran. (see Paul E. Smith "The effect of urea on the morphology of NaCl crystals: A combined theoretical and simulation study