Molecular Crystals

crystals formed by molecules bonded by weak van der Waals’ forces or by a hydrogen bond. A stronger covalent bond acts between the atoms within molecules. The phase transformations of molecular crystals—melting, sublimation, and polymorphic transitions—usually take place without the destruction of individual molecules.

Most molecular crystals are crystals of organic compounds. Naphthalene is a typical molecular crystal. Molecular crystals are also formed by certain simple substances (H₂, the halogens, N₂, O₂, and S₈), binary compounds such as H₂O, CO₂, N₂O₄, organometallic compounds, and certain complex compounds. The crystals of polymers, and also of proteins and nucleic acids, are also molecular crystals. Crystals of solidified inert gases, in which van der Waals’ forces bond atoms rather than molecules, are a special case of molecular crystals.

Low melting points, high coefficients of thermal expansion, high compressibility, and low hardness are characteristic of typical molecular crystals. Under ordinary conditions most molecular crystals are dielectrics. Some molecular crystals, such as the organic dyes, are semiconductors.