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The existence of different structural forms of the same chemical element. Diamond and graphite are crystalline examples for carbon; molecular oxygen and ozone illustrate that allotropy need not consist only of different packing arrangements of identical molecules.

In crystal polymorphism, the same chemical composition has different crystal structures; for an chemical element, this is a form of allotropy. Diamond has a three-dimensional tetrahedral network of covalent bonds, giving high hardness and a wide band gap. Graphite has strongly bonded planar sheets, with weaker interactions between sheets. Sliding between sheets makes it a useful solid lubricant; mobile electronic states give good electrical conduction within the sheets. Their different properties come from different bonding and structures, despite both being carbon.