allotropy (əlŏˈtrəpē) [key] [Gr., = other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes. Allotropes generally differ in physical properties such as color and hardness; they may also differ in molecular structure or chemical activity, but are usually alike in most chemical properties. Diamond and graphite are two allotropes of the element carbon. Ozone is a chemically active triatomic allotrope of the element oxygen. Phosphorus, sulfur, and tin also exhibit allotropy. Many metals have allotropic crystalline forms that are stable at different temperatures. Polymorphism is an analogous phenomenon observed in chemical compounds.