What are Molecular crystals?
Molecular crystals in which the units occupying the lattice points are molecules.
The lattice points in molecular crystals consist of specific molecules which do not carry any charge. Two types of force binding the molecules are ; (i) Dipole-dipole forces and (ii) van der Waals forces. Dipole-dipole forces in solid consist of a polar molecule. As shown in the figure, the negative end of one molecule attracts the positive end of a neighbouring molecule in the case of water molecules.
Dipole-dipole forces of attraction between water molecules
The van der Waals forces are more general and occur in molecular crystals. Both molecular forces are much weaker than the coulombic forces of attraction between oppositely charged ions in ionic crystals. The binding energy of the molecular crystal is less than in ionic crystals. The heat of vaporization means that the energy required to separate molecules from one another in a molecular crystal is very low compared to the value for ionic or covalent crystals. The molecular crystals are more volatile and have lower melting and boiling points.
With weak forces binding the molecules together, molecular crystals are usually soft, easily compressible and easily distorted. In molecular crystals, no ions or charged particles are present, which are bad conductors of electricity in solids, liquids, and dissolves.
Examples of molecular crystals are NH3, H2O and CO2.
