It is a tetravalent element as it can share its four valence electrons to form four bonds with another element.
Forms:
Graphite: There is a layer of the Carbon atom that shares a covalent bond with three other Carbon atoms forming a hexagonal structure. It is black, shiny, opaque, and slippery material.
Diamond: The Carbon atoms are arranged in a three-dimensional structure through a covalent bond with the other four Carbon atoms. It is a colorless, transparent, and lustrous form of Carbon with a high melting point.
Fullerene: All the Carbon atoms are arranged in the hexagonal shape of a sphere. Carbon nanotubes are cylindrical fullerene.