An empirical classification of solid types

There is no unique way of classifying solids. The method chosen depends in great measure on the purpose at hand. For the present we will classify solids according to the type of bond that holds the constituent particles of the solid together. 

In the first three solid types (metals, ionic crystals, van der Waals crystals), the forces of interaction that hold the particles together do not act in any preferred direction in covalent crystals, the bonds are formed only in special directions because of the directional character of the covalent bond. The principles governing the direction of bond formation in covalent crystals are the same as those governing the covalent bond in molecules. 

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