Molecular crystals

Substances such as $Cl_2$, $CH_4$, $H_2$, $CO_2$ and $O_2$ give molecular crystals on solidification, and crystalline organic compounds are molecular solids as well. Their characteristics:

i. The constituent particles of molecular solids are molecules (or unbonded single atoms) of the same substance.

ii. The bonds within the molecules are covalent; the molecules themselves are held together by various intermolecular forces of attraction, of three kinds:

a. Weak dipole-dipole interactions in polar molecules that possess a permanent dipole moment, such as solid HCl, $H_2O$ and $SO_2$.

b. Very weak dispersion or London forces in nonpolar molecules such as solid $CH_4$ and $H_2$. These forces also hold monoatomic solids like argon and neon together. (These substances are usually gases at room temperature.)

c. Intermolecular hydrogen bonds in solids such as $H_2O$ (ice), $NH_3$, HF and so forth. …