Write a note on the allotrophy of carbon.

Explanation

The existence of an element in different physical forms by possessing similar chemical properties is known as allotropy.
Carbon has many allotropic forms. They are of both crystalline and amorphous. The crystalline allotropes of carbon are diamond, graphite and fullerenes.
Diamond : In diamond each carbon atom undergoes `sp_3` hybridisation and is in bond with four other carbon atoms in a tetrahedral form. This structure extends in space and produces a rigid three dimensional network of carbon atoms. These bonds are present throughout the lattice. To break all these extended bonds large amount of energy is required. So diamonds are very hard and have high melting point. They used as precious stones, abrasives, cutting tools etc.
Graphite : It has layered lattice structure in which every carbon is in `sp_2` hybridisation and in bond with three carbon atoms. The carbon atoms are arranged in sheets containing hexagonal rings. The fourth electron is in `pi` bond and delocalised on all the carbon atoms in a sheet. The distance between the layers is 340 pm. They are held together by van der Waals forces. They are slippery in nature. Hence graphite can be used as solid lubricant. Due to delocalised electrons graphite act as a good electrical conduction.
Fullerene: is a crystalline allotrope of carbon. Fullerenes are made by heating graphite in an electric arc in the presence of inert gas. The soot formed by condensation of vapour of carbon contains mainly `C_(60)` fullerene molecules. It is known as Buckminster fullerene. It has soccer ball shape.
`C_(60)` fullerene contains 20 six membered rings and 12 five membered rings. A six membered ring is fused with either six membered ring or with five membered ring but a five membered ring is always fused with six membered ring. All carbon atoms in fullerene are in `sp^2` hybridisation and are in bond with three other carbon atoms. The fourth electron of carbon delocalises on all carbon atoms giving aromatic character to fullerene. These are also called bucky balls, in short.