Allotropy

C graphite diamond [60]fullerene glassy carbon	N	O dioxygen ozone
Si	P white phosphorus black phosphorus red phosphorus	S α-sulfur β-sulfur γ-sulfur ε-sulfur
Ge	As α-arsenic yellow arsenic ε-arsenic amorphous arsenic	Se α-selenium β-selenium γ-selenium grey selenium red selenium black selenium
Sn white tin grey tin	Sb α-antimony antimony-I antimony-II	Te
Pb	Bi α-bismuth ζ-bismuth	Po α-polonium β-polonium
Some allotropes of the elements of groups 14, 15 and 16. The most stable allotrope at 25 °C and 1 bar is listed first; amorphous allotropes are listed in italics.

Allotropy is a phenomenon in chemistry whereby a given chemical element can exist in several different structural forms. Each of these forms is called an allotrope. The best known example is that of diamond and graphite, which are both allotropes of carbon. Allotropy can be seen as a special case of polymorphism.

The term was first proposed by Jöns Jakob Berzelius in 1841: it is derived from the Greek άλλοτροπἱα (allotropia; variation, changeableness), from ἄλλος (allos; other, different) and τρόπος (tropos; turn of mind, manner).

At any given temperature and pressure, one allotrope will be more thermodynamically stable than the others. Nevertheless, other allotropes can exhibit substantial kinetic stability, e.g. diamond: such allotropes are said to be metastable. Allotropy is most pronounced in groups 14, 15 and 16 of the periodic table. Boron also forms several allotropes: the different structural forms of the d- and f-block metals are usually referred to as phases rather than allotropes.

Nomenclature

Most allotropes have long-accepted semisystematic names (e.g., red phosphorus, white phosphorus) or trivial names (e.g., ozone, graphite). The different allotropes may also be labelled with Greek letters (e.g., α-sulfur, β-sulfur), where the α-allotrope is usually (but not always) the most stable under ambient conditions. A systematic nomenclature is available, but is not widely used.