Allotrope

Allotropes are different forms of an assemblage of the atoms of a particular chemical element in the same physical state (gas, liquid, or solid), the different forms resulting from different configurations of the assemblage making up its molecular or crystalline structure.

The existence of an element in more than one form is known as allotropy. In accord with the definition of an allotrope, allotropy does not extend to include different forms of an element purely because of a difference of physical state, so that, for example, liquid nitrogen and gaseous nitrogen do not qualify as allotropes of that element.

Examples of allotropes are the two well-known forms of oxygen: dioxygen (O₂), which forms roughly one fifth of the atmosphere, and ozone (O₃), also present in the upper atmosphere but in far smaller amount, which absorbs harmful ultraviolet light from the Sun, and also acts as a greenhouse gas.

Carbon has more than 40 allotropes, most of them amorphous. Among the crystalline allotropes is graphite (where the atoms are arranged in hexagonal layers) and diamond (where the atoms form a three-dimensional tetrahedral structure). Of this pair graphite is the more stable, but fortunately at normal temperatures the rate of conversion from diamond to graphite is imperceptibly slow! In the mid-1980s a further family of carbon allotropes known as fullerenes (or "buckyballs") were discovered, leading to the award of the 1996 Nobel Prize in chemistry to Harold Kroto, Robert Curl and Richard Smalley.

Other elements which exhibit allotropy include tin, which exists in grey nonmetallic and white metallic forms, phosphorus, which has white, red and black forms, and sulfur, with a number of allotropes, including rhombic, monoclinic, and plastic amorphous forms.

The German chemist Eilhard Mitscherlich first discovered allotropy, in sulfur.

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