Allotropy

Allotropy is the property of certain chemical elements to exist in two or more distinct structural forms in the same phase of matter. The alternative forms, known as allotropes, arise from differences in how atoms are bonded or arranged in the solid, liquid, or gaseous state, and they typically display markedly different physical and chemical properties. Allotropy is most commonly observed in solids and often depends on pressure and temperature; some allotropes are stable only within specific conditions and can transform when those conditions change.

Examples of allotropy include carbon, which has diamond and graphite as well as other forms such as graphene and fullerenes; phosphorus, which exists as white, red, and black allotropes; oxygen, which occurs as diatomic O2 and triatomic O3; sulfur, which forms rhombic (alpha) and monoclinic (beta) allotropes; and tin, which exhibits white tin and gray tin, the latter appearing at lower temperatures.

Iron shows allotropy with body-centered cubic alpha-iron at room temperature and face-centered cubic gamma-iron at higher temperatures, a transformation with consequences for magnetism and steel properties.

Allotropy differentiates elements from polymorphism of compounds; it reflects how a single element can adopt multiple crystal structures or molecular arrangements, each with its own set of properties.