There are four types of solids. They all form crystals that have definite shape and volume, are virtually incompressible, and do not flow readily. However, properties such as hardness, melting point, mechanical characteristics, and conductivity vary.
Ionic crystals are formed from a combination of a metal cation and a nonmetal anion, for example NaCl(s). They form a three-dimensional crystal lattice: a regular, repeating pattern in the arrangement of the ions. The positive and negative ions are held together by strong, directional ionic bonds. Ionic crystals are hard, brittle, conductive as liquids and in solution (not as solids), and have high melting points.
Metallic crystals are formed from metal elements. The metal elements become cations in a three-dimensional arrangement, and they are held together by strong, nondirectional bonds created by the “sea” of mobile electrons. They are shiny, conductive, vary from soft to hard, have varying melting points, and they are malleable, ductile, and flexible. A Rice Krispies square is a good analogy for a metallic crystal: the grains of rice are metal cations, and the marshmallow represents the electron sea.
Molecular crystals are formed by nonmetal elements or molecular compounds packed closely together. This three-dimensional arrangement of neutral molecules is held together by relatively weak intermolecular forces. Molecular crystals are not very hard, are not conductors, have low melting points, and are packed together as closely as size and shape allows.
Covalent network crystals are formed from metalloids or carbon. These atoms are all connected in a network of covalent bonds, as if the entire solid was a single molecule. Specifically, the atoms are arranged in a three-dimensional network and are held together by strong, directional covalent bonds. These crystals are very hard, insoluble nonconductors, and they have very high melting points. Diamond, one form of C(s), is an example of a covalent network crystal.