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The Collaborative International Dictionary
Allotropy

Allotropism \Al*lot"ro*pism\, Allotropy \Al*lot"ro*py\, n. [Gr. ? other + direction, way, ? to turn: cf. F. allotropie.] (Chem.) The property of existing in two or more conditions which are distinct in their physical or chemical relations.

Note: Thus, carbon occurs crystallized in octahedrons and other related forms, in a state of extreme hardness, in the diamond; it occurs in hexagonal forms, and of little hardness, in black lead; and again occurs in a third form, with entire softness, in lampblack and charcoal. In some cases, one of these is peculiarly an active state, and the other a passive one. Thus, ozone is an active state of oxygen, and is distinct from ordinary oxygen, which is the element in its passive state.

Wiktionary
allotropy

n. (context chemistry English) A property, exhibited by some elements of existing in multiple forms with different atomic structures.

WordNet
allotropy

n. the phenomenon of an element existing in two or more physical forms [syn: allotropism]

Wikipedia
Allotropy

Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements. Allotropes are different structural modifications of an element; the atoms of the element are bonded together in a different manner. For example, the allotropes of carbon include diamond (where the carbon atoms are bonded together in a tetrahedral lattice arrangement), graphite (where the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (where the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations). 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 phase (i.e. different solid, liquid or gas forms); these different states are not, themselves, considered to be examples of allotropy.

For some elements, allotropes have different molecular formulae which can persist in different phases – for example, two allotropes of oxygen ( dioxygen, O, and ozone, O), can both exist in the solid, liquid and gaseous states. Conversely, some elements do not maintain distinct allotropes in different phases – for example phosphorus has numerous solid allotropes, which all revert to the same P form when melted to the liquid state.