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Allotropy is the property of certain chemical elements to exist in two or more distinct structural forms, called allotropes, in the same physical state. The different forms arise from variations in the arrangement of atoms or in the bonding between them, leading to notable differences in properties such as hardness, density, electrical conductivity, and reactivity. Allotropy is particularly common among nonmetals and metalloids, and also occurs in some metals under varying temperature and pressure.

Common examples include carbon, which forms diamond (three-dimensional network of strong covalent bonds) and graphite (sheets of bonded carbon atoms arranged in a plane). Graphite is soft and conductive, while diamond is extremely hard and an excellent insulator. Phosphorus exists as white, red, and black allotropes, each with different stability and reactivity. Sulfur has several allotropes, including orthorhombic and monoclinic forms, with distinct melting points and crystal structures. Oxygen is notable for O2 and O3 (ozone), both allotropes of the element, with ozone being far more reactive. Tin shows metal allotropy, with white tin and gray tin forming under temperature change, a transformation historically associated with the “tin pest.” Iron exhibits allotropy as well, with different crystalline forms (alpha-iron and gamma-iron) that are stable at different temperatures and influence metallurgical properties.

Allotropic forms are typically driven by changes in temperature, pressure, or both, and the transitions can be reversible under appropriate conditions. Allotropy is distinct from chemical isomerism, which involves different arrangements of atoms in molecules rather than different structural forms of the same element. Understanding allotropy helps explain variations in material properties and guides applications in chemistry, geology, and materials science.