Allotropy

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In chemistry, allotropy (adjective allotropic) means the existence, especially in the solid state, of two or more crystalline or molecular structural forms of an element. An element can exist in several forms or modifications that not only have strongly different outer appearances, but also have different physical properties. Yet, chemically it is one and the same element. The classic example is formed by the three allotropes diamond, graphite and charcoal, which consist of pure carbon and burn in the presence of oxygen, giving carbon dioxide. Diamond is extremely hard and often transparent, graphite is soft, flaky, and black, and charcoal is brittle and black.

Another example is rhombic and monoclinic sulphur, which are two different crystal structures of the element sulphur.

Allotropy must be distinguished from the existence of different phases of the same substance, liquid, solid, or gas.

Allotropy was discovered in 1821 for sulphur by the German chemist Eilhard Mitscherlich.

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Glossary California Air Resources Board