Question

Describe briefly allotropism in p-block elements with specific reference to carbon.

Answer 1

Some elements exist in more than one crystalline or molecular form in the same physical state. This phenomenon is called allotropism.

The most common allotropes of carbon are,

1. Graphite
2. Diamond
3. Fullerenes
4. Carbon nanotubes
5. Graphene

1. Graphite:

(i) It is the most stable allotropic form of carbon at normal temperature and pressure.

(ii) It is soft and conducts electricity.

(iii) It is composed of flat two-dimensional sheets of carbon atoms.

(iv) Each sheet is a hexagonal net of sp² hybridised carbon atoms with a C – C bond length of 1.41 Å.

       Structure of graphite

2. Diamond:

(i) It is very hard.

(ii) The carbon atoms in diamond are sp¹ hybridised, with a C – C bond length of 1.54 Å.

(iii) In the diamond, carbon atoms are arranged in a tetrahedral manner.

        Structure of Diamond

3. Fullerenes:

(i) It is a newly synthesised allotrope of carbon.

(ii) The C₆₀ molecules have a soccer ball-like structure and is called buckminster fullerene or buckyballs.

(iii) It has a fused ring structure consists of 20 six-membered rings and 12 five-membered rings.

(iv) Each carbon atom is sp² hybridised.

(iv) The C – C bond distance is 1.44 A and the C = C distance is 1.38 Å.

   Structure of Fullerenes

4. Carbon nanotubes:

(i) It is recently discovered allotropes, have graphite-like tubes with fullerene ends.

(ii) These nanotubes are stronger than steel and conduct electricity.

      Structure of carbon nanotubes

5. Graphene:

It has a single planar sheet of sp² hybridised carbon atoms that are densely packed in a honeycomb crystals lattice.

         Structure of graphene