Write a note on interstitial compounds.

Interstitial compounds are those which are formed when small atoms like $B$, $H$, $C$ or $N$ are trapped inside the crystal lattice of metals.

The transition elements form interstitial compounds with these elements. The $B$, $H$, $C$ and $N$ occupy the vacant spaces in the crystal lattice and as a result, the compounds formed are extremely hard.

They are classified as borides, hydrides, carbides or nitrides depending on nature of atoms trapped in the vacant sites. They are usually non-stoichiometric, neither typically covalent nor ionic. Ex. : $\mathrm{TiC}, \mathrm{Mn}_{4} \mathrm{~N}, \mathrm{Fe}_{3} \mathrm{H}, \mathrm{VH}_{0.58}$ and $\mathrm{TiH}_{1.7}$ etc. The important physical and chemical characteristics of these compounds are :

$(i)$ High melting points and even higher than pure metals.

$(ii)$ They are extremely hard. Some borides approach diamond in hardness.

$(iii)$ They retain metallic conductivity.

$(iv)$ They are chemically inert.