Allotropy (Gr. allos, other, and tropos, manner), a name applied by Jons Jacob Berzelius to the property possessed by certain substances of existing in different modifications; the various forms are known as allotropes. Jons Jacob Berzelius used the name in an entirely different sense (see Macmillan Encyclopedia of Chemistry, edited by J.J.Lagowski, 1997, Simon Schuster).

Some classic examples of elements that have allotropes are phosphorus (in "red" and "white" and "purple" etc. forms), oxygen (O₂ and O₃) and carbon (in the form of graphite, diamond, fullerenes, and others - see allotropes of carbon). The term allotropes may also be used to refer to the molecular forms of an element (such as a diatomic gas), even if there is only one such additional form.

Sulfur is an additional example of an element with several allotropic forms. Amorphous (plastic sulfur) is produced by quickly cooling the crystalline form, generating helical structure with eight atoms per spiral.

Allotropy specifically refers to the chemical bond structure between atoms of the same kind and should not be confused with the existence of multiple physical states, such as with water, which can exist as a gas (steam), a liquid (water), or a solid (ice). These phases of water are not allotropes, since they are caused by changes in the physical bonding between water molecules, rather than changes in the chemical bonding of the water molecules themselves. Allotropes of an element can be in any state, gaseous, liquid, or solid.

Allotropy usually refers to pure elemental solids, while polymorphism may refer to elemental solids or more generally to any material having multiple crystal structures.

As can be seen with the example of carbon allotropes, certain physical properties can vary dramatically from allotrope to allotrope. In diamond, carbon atoms are connected each to four other carbon atoms in a tetrahedal lattice structure, whereas in graphite, each carbon atom is firmly bonded to just three other carbon atoms in hexagonal sheets. These hexagonal sheets are then more loosely coupled to one another in stacks. The structure of fullerenes (a carbon allotrope found in soot) resembles that of graphite, except that instead of hexagons of carbon atoms, smaller regular polygons are formed, such as a mix of hexagons and pentagons, such that the sheet can fold back onto itself into closed spheroids, as with the seams of a soccer ball. Allotropes not only show dramatic differences in physical properties but also show differences in chemical properties. Graphite can be oxidized by nitric acid to give compounds related to benzene whereas diamond does not give compounds related to benzene.