Editing Allotropy (section)

{{short description|Property of some chemical elements to exist in two or more different forms}}
{{distinguish|Xenophagy{{!}}Allotrophy}}

[[File:Diamond and graphite.jpg|thumb|193x193px|[[Diamond]] and [[graphite]] are two allotropes of carbon: pure forms of the same element that differ in crystalline structure.]]

'''Allotropy''' or '''allotropism''' ({{ety|grc|''ἄλλος'' (allos)|other||''τρόπος'' (tropos)|manner, form}}) is the property of some [[chemical element]]s to exist in two or more different forms, in the same physical [[State of matter|state]], known as '''allotropes''' of the elements. Allotropes are different structural modifications of an element: the [[atom]]s of the element are [[Chemical bond|bonded]] together in different manners.<ref>{{GoldBookRef|title=Allotrope|file=A00243|accessdate=August 11, 2015}}</ref>
For example, the [[allotropes of carbon]] include [[diamond]] (the carbon atoms are bonded together to form a [[Cubic crystal system|cubic lattice]] of [[Tetrahedral molecular geometry|tetrahedra]]), [[graphite]] (the carbon atoms are bonded together in sheets of a [[hexagonal lattice]]), [[graphene]] (single sheets of graphite), and [[fullerene]]s (the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations).

The term ''allotropy'' is used for elements only, not for [[Chemical compound|compounds]]. The more general term, used for any compound, is [[Polymorphism (materials science)|polymorphism]], although its use is usually restricted to solid materials such as crystals. Allotropy refers only to different forms of an element within the same physical [[State of matter|phase]] (the state of matter, such as a [[solid]], [[liquid]] or [[gas]]). The differences between these states of matter would not alone constitute examples of allotropy. Allotropes of chemical elements are frequently referred to as ''[[Polymorphism (materials science)|polymorphs]]'' or as ''[[Phase (matter)|phase]]s'' of the element.

For some elements, allotropes have different molecular formulae or different crystalline structures, as well as a difference in physical phase; for example, two [[allotropes of oxygen]] ([[Oxygen|dioxygen]], O<sub>2</sub>, and [[ozone]], O<sub>3</sub>) can both exist in the solid, liquid and gaseous states. Other elements do not maintain distinct allotropes in different physical phases; for example, [[phosphorus]] has [[Allotropes of phosphorus|numerous solid allotropes]], which all revert to the same P<sub>4</sub> form when melted to the liquid state.