Molecular Solid - Properties

Properties

Weakness of intermolecular forces results in low melting temperatures of molecular solids. Whereas the characteristic melting point of metals and ionic solids is ~1000 °C, most molecular solids melt well below ~300 °C (see table), thus many corresponding substances are either liquid (ice) or gaseous (oxygen) at room temperature. Molecular solids also have relatively low density and hardness. This is because of the light elements involved and relatively long and thus weak intermolecular bonds. Because of the charge neutrality of the constituent molecules and long distance between them, molecular solids are electrical insulators.

The above tendencies can be illustrated on example of different allotropes of phosphorus. White phosphorus, a molecular solid, has a relatively low density of 1.82 g/cm3 and melting point of 44.1 °C; it is a soft material which can be cut with a knife. When it is converted to the covalent red phosphorus, the density increases to 2.2–2.4 g/cm3 and melting point to 590 °C, and when white phosphorus is transformed into the (also covalent) black phosphorus, the density becomes 2.69–3.8 g/cm3 and melting temperature ~200 °C. Both red and black phosphorus forms are significantly harder than white phosphorus, and whereas white phosphorus is an insulator, the black allotrope, which consists of layers extending over the whole crystal, does conduct electricity.

Conductivity of molecular solids can be illustrated on example of fullerene. Its solid is an insulator because all valence electrons of carbon atoms are involved into the covalent bonds within the individual carbon molecules. However, inserting (intercalating) alkali metal atoms between the fullerene molecules provides extra electrons, which can be easily ionized from the metal atoms and make material conductive and even superconductive.