Molecular solids occur when covalent molecules are held together by intermolecular forces. Typically, these types of solids have lower melting and boiling points than metallic, network atomic, or ionic solids, because the intermolecular forces holding the molecules together are weaker. These solids contain molecular units at each lattice position; for example, ice contains water molecules at its lattice positions.
Effect of Intermolecular Forces
Molecular solids are characterized by strong covalent bonding within the molecules and relatively weak forces between the molecules. However, the forces that exist among the molecules in a molecular solid depend on the nature of the molecules. Molecules like carbon dioxide or iodine do not have any dipole moment so their intermolecular forces are London dispersion. Because these forces are relatively weak, the substances are expected to be in gaseous form at 25°C. However, as the size of the molecules increases, the London forces also increase causing other substances to be in a solid form at 25°C.