Allotropes of Carbon
Carbon exists in various allotropic forms of which graphite and diamond are the well known crystalline forms.
Diamond possesses a rigid structure in which the carbon atoms are covalently bonded in a tetrahedral fashion with C-C bond length 154 pm. It is very hard due to which it is used as an abrasive.
Graphite posses a layer structure with carbon atoms in the form of planar hexagonal rings , it undergoes sp2 hybridisation in which carbon atoms are linked through three sigma bonds and one pi bond. The successive layers are 340 pm far apart and C-C bond length is 141.5 pm. Due to lose packing of layers covalent bonds can be easily broken unlike diamond. Hence, graphite is a good conductor of heat and electricity.
Giant modern structures of carbon were formed when an electric arc is struck in graphite in an inert atmosphere. This structure containing 60 or 70 carbon atoms forming cage like structures are called fullerenes . They are the purest forms of allotropic carbon. C60 which has a shape like soccer ball and called Buckminster fullerene.