The phenomenon of various forms of an element having different physical properties but similar chemical properties is called allotropy. The various forms are called allotropes.

Allotropes of carbon: Except diamond, graphite, fullerenes and graphene, all other forms are amorphous allotropes of carbon.

Diamonds are purest of natural carbon. In diamond, carbon is sp³ hybridized and its symmetry is tetrahedral. Each carbon atom is linked to 4 carbon atoms by strong covalent bonds. Here, each carbon atom lies at the center of a regular tetrahedral and four other carbon atoms are present at the corners of a tetrahedral. In Diamond all C–C bond lengths are equal to 1.54Å. It is a non–conductor of electricity as it has no free electrons.

In graphite, carbon is sp² hybridized and its symmetry is hexagonal. Each carbon atom is linked to three other carbon atoms by covalent bonds to form hexagonal rings having C–C bond length 1.42Å. The hexagonal rings form huge sheets or layers, which are held together by weak van der waal forces. The distance between layers is 3.42Å. It is a good conductor of electricity as it has free electrons.

Fullerene is crystalline allotropic form of carbon and is composed entirely of carbon, in the form of a hollow sphere, ellipsoid or tube. Spherical fullerenes are also called bucky balls and Cylindrical ones are called carbon nanotubes or bucky tubes.
Graphene is an allotrope of carbon, its structure is one atom thick planar sheets of sp² bonded carbon atoms that are densely packed in a honeycomb crystal lattice.
Coal, Coke, Gas carbon, Wood – CharCoal, Sugar Charcoal, Bone Charcoal are some of the amorphous allotropes of carbon.

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