Question
Describe briefly allotropism in p-block elements with specific reference to carbon.
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Answer
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,
- Graphite
- Diamond
- Fullerenes
- Carbon nanotubes
- Graphene
(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 $sp^2$ 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 $sp^1$ 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 $C_{60}$ 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 $sp^2$ 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 $sp^2$ hybridised carbon atoms that are densely packed in a honeycomb crystals lattice.
Structure of graphene
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