In short
Allotropes are different forms of the same element in the same physical state, with the atoms joined in different ways. Diamond and graphite are allotropes of carbon.
The atoms are identical, but the arrangement is not, so the physical properties differ. In diamond each carbon atom bonds to four others in a rigid network, so it is very hard and does not conduct. In graphite each carbon atom bonds to three others in flat layers, with free electrons between them, so it is soft and conducts electricity. Other allotropes of carbon are fullerenes, graphene and nanotubes. Oxygen () and ozone () are allotropes too.
Burning of diamond or of graphite in oxygen gives only carbon dioxide, which shows that both are made of carbon alone.
Also searched as: allotropes, allotropes of carbon, what is an allotrope.
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