Question

Its said that diamond and graphite are allotropes of carbon. what do you mean by allotrope???

Answer 1

same element existing in different physical form called allotropy.
diamond and graphite are basically both carbon but have totally different physical properties .

Answer 2

The various physical forms of an element in which it can exist are called allotropes of that element. Carbon can exist in 3 allotropic forms:diamond, graphite and buckminsterfullerene.