allotropy (əlŏ´trəpē) [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.

ozone Unstable, pale-blue, gaseous allotrope of oxygen, formula (O³). It has a characteristic pungent odour and decomposes into molecular oxygen. It is present in the atmosphere, mainly in the ozone layer. Prepared commercially by passing a high-voltage discharge through oxygen, ozone is used as an oxidizing agent in bleaching, air-conditioning and purifying water. See also allotropy

allotropy Property of some chemical elements that enables them to exist in two or more distinct physical forms. Each form (an allotrope) can have different chemical properties but can be changed into another allotrope – given suitable conditions. Examples of allotropes are molecular oxygen and ozone, white and yellow phosphorous, and graphite and diamond (carbon).