Question
Write a note on ‘Allotropes of carbon’.
✓
Answer
- The element carbon exists in nature in different forms with different physical properties, but identical chemical properties.
- These forms are called allotropes of carbon.
- Carbon possesses three allotropes :
- $(1)$ Diamond, $(2)$ Graphite and $(3)$ Fullerene.
- These three allotropes consist of pure carbon.
- Diamond, graphite or fullerene when burnt in air, form carbon dioxide gas, which is colourless and odourless and turns lime water milky, Hence, chemical properties of these allotropes are same.
- $(1)$ Diamond:
- $(a)$ Structure: [$2$ Marks]
- In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional tetrahedral structure.
- [$(a)$ Tetrahedral arrangement of carbon atoms in diamond $(b)$ Three dimensional structure of diamond]
- $(b)$ Physical properties of diamond: [$2$ Marks]
- Carbon atoms are arranged tetrahedrally in diamond; and it is the hardest natural substance known.
- Carbon atoms are closely packed in diamond, hence, it has a high density. ( $3.51 g cm ^{-3}$ )
- Diamond possess numerous covalent bonds, hence, its melting point is quite high. ($4203 \ K$)
- There are no free electrons electrons in a diamond crystal, therefore, diamond is a non-conductor of electricity.
- Diamonds are transparent because of their high refractive index $(2.5).$
- Diamonds can be synthesised by subjecting pure carbon to very high pressure and temperature.
- These synthetic diamonds are small but are indistinguishable from natural diamonds.
- $(2)$ Graphite:
- $(a)$ Structure :
- In graphite, each carbon atom is bonded to three other carbon atoms in the same plane giving a hexagonal array.
- One of these bonds is a double bond, and thus the valency of carbon is satisfied.Graphite structure is formed by the hexagonal arrays being placed in
- layers one above the other.
- [$(a)$ The structure of graphite $(b)$ Hexagonal arrangement of layers]
- $(b)$ Physical properties of graphite:
- Graphite is smooth and slippery.
- The density of graphite is lower than diamond. $\left(2.22 g cm ^{-3}\right)$.
- In graphite, three valence electrons are used for bond formation and hence, one valence electron is free to move.
- As a result, graphite is a good conductor of heat and electricity.
- $(3)$ Fullerene: [$2$ Marks]
- Fullerenes form another class of carbon allotropes.
- The first one to be identified was $C -60$, which has carbon atoms arranged in the shape of football.
- Since this looked like the geodesic dome designed by the US architect Buckminster Fuller, the molecule was named fullerene
[The structure of C - 60 Buckminster Fullerene]
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