Allotropy, << uh LOT ruh pee, >> in chemistry, is the ability of an element to exist in more than one form. These forms are called allotropes. For example, three common allotropes of carbon are: (1) hard, transparent diamond crystals, (2) soft, steel-gray or black graphite crystals, and (3) black, sooty, uncrystallized carbon black (see Carbon ). The word allotropy comes from two Greek words meaning another and way.
Solid allotropes differ in their crystal structures. But allotropic gases differ in their molecular structures. For example, each molecule of ordinary oxygen is made of two oxygen atoms. Each molecule of the allotrope ozone is made of three oxygen atoms. Ordinary oxygen has no odor. But ozone has a peculiar sharp odor (see Ozone ).
