Allotrope Definition and Examples

Including the difference between allotropism and polymorphism

Different forms of carbon
Allotropes of carbon including a coal, charcoal, graphite, and diamonds. Dave King / Getty Images

The term allotrope refers to one or more forms of a chemical element that occur in the same physical state. The different forms arise from the different ways atoms may be bonded together. The concept of allotropes was proposed by Swedish scientist Jons Jakob Berzelium in 1841. The ability for elements to exist in this way is called allotropism.

Allotropes may display very different chemical and physical properties. For example, graphite and diamond are both allotropes of carbon that occur in the solid state. Graphite is soft, while diamond is extremely hard. Allotropes of phosphorus display different colors, such as red, yellow, and white. Elements may change allotropes in response to changes in pressure, temperature, and exposure to light.

Examples of Allotropes

To continue the carbon example, in diamond, the carbon atoms are bonded to form a tetrahedral lattice. In graphite, the atoms bond to form sheets of a hexagonal lattice. Other allotropes of carbon include graphene and fullerenes.

O2 and ozone, O3, are allotropes of oxygen. These allotropes persist in different phases, including the gas, liquid, and solid states.

Phosphorus has several solid allotropes. Unlike the oxygen allotropes, all phosphorus allotropes form the same liquid state.

Allotropism Versus Polymorphism

Allotropism refers only to the different forms of pure chemical elements. The phenomenon in which compounds display different crystalline forms is called polymorphism.