In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid threedimensional structure. This makes diamond the hardest substance known whereas in graphite, each carbon atom is bonded to three other carbon atoms in same plane giving a hexagonal array being placed in layers one above the other this makes graphite a smooth and slippery substance. Thus, both diamond and graphite are composed of carbon, therefore their chemical properties are same and they are the allotropic forms of carbon. An allotrope is another form of the same element having different physical properties but the same chemical properties.
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Elements forming ionic compounds achieve noble gas configuration by either gaining or losing electrons from the outermost shell. In the case of carbon, it has four electrons in its outermost shell and needs to gain or lose four electrons to attain noble gas configuration. If it were to gain or lose electrons -
(i) It could gain four electrons forming C4− anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons, that is, four extra electrons.
(ii) It could lose four electrons forming C4+ cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons.
Carbon overcomes this problem by sharing its valence electrons with other atoms of carbon or with atoms of other elements and forms the covalent bonds.