property of some chemical elements to exist in two or more different forms
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Allotropy (or allotropism) is when a chemical element can exist in two or more different forms in the same physical state or phase. These different forms are called allotropes. Therefore, an allotrope is a different structure in which an element appears.[1] This happens when the atoms of the element are bonded together in a different way.

Graphite is one allotrope of carbon
Diamond is another allotrope of carbon

For example, the allotropes of carbon include:

Only some elements have allotropes.

The term allotropy is used for elements only, not for compounds. The more general term, used for any crystalline material, is polymorphism. Allotropy refers only to different forms of an element within the same state (i.e., different solid, liquid or gas forms); these different states are not, themselves, considered examples of allotropy.