Allotropy
Allotropy, the property that certain chemical elements have of existing in two or more different forms in the same physical state (gas, liquid, or solid). Some allotropic forms, or allotropes, arise from differences in the number of atoms in a molecule of the element, as in ordinary oxygen, O2, and triatomic oxygen (ozone), O3. Allotropy can also be caused by different arrangements of atoms in amorphous (noncrystalline) or crystalline structures of an element. Carbon, for example, exists in strikingly different crystalline allotropic forms as diamond and as graphite. The property of crystallizing in two or more forms is termed polymorphism.
A number of other elements, notably tin, iron, selenium, phosphorus, and sulfur, have allotropes resulting from different arrangements of their atoms. For a description of some of these allotropes, see the individual articles on these elements.