What are allotropes? Sketch the structure of two allotropes of carbon family namely diamond and graphite. What is the impact of structure on physical properties of two allotropes?

Allotropes. Allotropes are defined as the different forms of an element which have different physical but similar chemical properties.

Both diamond and graphite are network covalent solids in which the carbon atoms are linked by covalent bonds.

In diamond: Each carbon atom is sp³ hybridised. It is bonded tetrahedrally to four other carbon atoms by simple covalent bonds. Thus, it has a three-dimensional network of strong covalent bonds in which C–C bond length is 154 pm and each bond angle is 109° – 28'. Due to sp³ hybridization, the carbon atoms in diamond are closely packed and all the valence electrons are involved in bonding, leaving no free electrons, so, diamond is very hard, bad conductor of electricity and has high melting point.

In graphite: Each carbon atom is sp² hybridised and is bonded to three other carbon atoms through covalent bonds forming hexagonal planar rings. The C–C covalent bond distance in rings is 142 pm. Graphite has a two-dimensional sheet-like structure. Due to sp² hybridization, one p-unhybridised orbital of the carbon atom in graphite

is not involved in bond formation. Thus, one valence electron of each carbon atom is free to move (mobile electron) from one point to other and this accounts for the electrical conductivity and soft nature of graphite.