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The existence of one element in different forms, having different physical properties, but similar chemical properties is known as allotropy. Carbon shows allotropy. Such different forms are called 'allotropes' of an element or allotropic forms. The various allotropic forms of carbon can be broadly classified into two classes.
Crystalline form: Diamond, Graphite
Amorphous form: Coal, Coke, Charcoal (or wood charcoal), Animal Charcoal (or bone black), Lamp black, Carbon black, Gas carbon and Petroleum coke.
Diamonds and graphite are two crystalline allotropes of carbon. Diamond and graphite both are covalent crystals. But, they differ considerably in their properties.
It occurs naturally in free state.
It is the hardest natural substance known.
It has high relative density (about 3.5).
It is transparent and has high refractive index (2.45).
It is non-conductor of heat and electricity.
It burns in air at 900°C to give CO2.
It occurs as octahedral crystals.
It is insoluble in all solvents.
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It occurs naturally and is manufactured artificially.
It is soft and greasy to touch.
Its relative density is 2.3.
It is black in colour and opaque.
Graphite is a good conductor of heat and electricity.
It bums in air at 700-800°C to give CO2.
It occurs as hexagonal crystals.
It is insoluble in all ordinary solvents
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These differences in the properties of diamond and graphite are due to the difference in their structures. In diamond, each C atom is linked to its neighbors by four single covalent bonds. This leads to a three-dimensional network of covalent bonds. In graphite, the carbon atoms are arranged in flat parallel layers as regular hexagons. Each carbon in these layers is bonded to three others by covalent bonds. Graphite thus acquires some double bond character. Each layer is bonded to adjacent layers by weak van der Waals forces. This allows each layer to slide over the other easily. Due to this type of structure graphite is soft and slippery, and can act as a lubricant. Graphite is also a good conductor of electricity due to mobile electrons in it.
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