Molecular Solids

In molecular solids the constituent particles are molecules. They are sub-divided into three types according to the kind of attraction between the molecules.

(i) Non-polar molecular solids

These are made of either single atoms (for example argon and helium) or molecules formed by non-polar covalent bonds (for example H$_2$, Cl$_2$ and I$_2$). The particles are held only by weak dispersion (London) forces. As a result these solids are soft, are non-conductors of electricity, and have low melting points — most of them are gases or liquids at room temperature and pressure.

(ii) Polar molecular solids

Here the molecules — such as HCl and SO$_2$ — are formed by polar covalent bonds and are held together by the somewhat stronger dipole–dipole interactions. They are also soft and non-conducting, but their melting points are higher than those of non-polar molecular solids. Even so, most are gases or liquids at room conditions; solid SO$_2$ and solid NH$_3$ are examples.

(iii) Hydrogen-bonded molecular solids …