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.
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Allotropy is the existence of an element in two or more different forms in the same physical state. These different forms, known as allotropes, have distinct physical and chemical properties despite being composed of the same atoms. The variations in structure can result from differences in bonding, arrangement, or spatial orientation of the atoms.