Allotrope

An allotrope is one of two or more distinct forms of a chemical element in the same physical state. These forms arise from differences in how atoms are arranged or bonded, creating variations in structure, bonding, and properties such as color, hardness, electrical conductivity, and reactivity. Allotropy is common for elements that can adopt more than one stable or metastable crystalline arrangement, and it is different from polymorphism, which describes different forms of compounds rather than the element itself.

Representative allotropes include carbon, which exists as diamond (a three-dimensional covalent network with exceptional hardness and high thermal conductivity), graphite (planar sheets with weaker interlayer bonds that conduct electricity along the planes), graphene and fullerenes, and amorphous carbon. Oxygen exists as O2 and O3, with ozone (O3) being more reactive and absorbing ultraviolet light. Phosphorus occurs as white, red, and black allotropes, each with distinct bonding and properties. Sulfur forms multiple allotropes, including cyclic S8 molecules and polymeric chains, differing in stability and electrical behavior. Tin shows alpha-tin and beta-tin forms, with low-temperature gray tin differing from the room-temperature metallic white tin.

All allotropes can interconvert under changes in temperature or pressure, and the study of allotropy informs materials science, geology, and chemistry.