Solids can be classified on the basis of the regular arrangements of constituent atoms, ions or molecules. There are two types of solids in this respect:
Those solids in which atoms, ions or molecules are arranged in a definite three-dimensional patterns with regular and repetitive characteristics are called crystalline solids.
The solids in which the structure unit that is atoms, ions or molecules are fixed in their position but are not regularly arranged are called amorphous solids.
The word amorphous means shapeless. The best examples are glass, plastics, rubber, glue etc. Many crystalline solids can be changed into amorphous solids by melting them and then cooling the molten mass rapidly. In this way the constituent particles do not find time to arrange themselves. Amorphous solids don’t have a sharp melting point that is why particles of glass soften over a temperature range and can be molded and blown into various shapes. It does not possess definite heat of fusion.