Allotropy
Allotropy, the manifestation by a chemical element of two or more distinct physical forms. Carbon, for example, displays allotropy in the forms of graphite, diamond, and buckminsterfullerene. Because of differing arrangements of atoms in their structures, allotropic forms of an element may exhibit greatly differing values for such physical properties as colour, lustre, density, hardness, odour, and electrical and thermal conductivity. Other elements displaying allotropy include phosphorus, sulphur, and tin. Carbon, symbol C, element that is crucial to the existence of living organisms, and that has many important industrial applications. The atomic number of carbon is 6; the element is in group 14 (or IVa) of the periodic table. Three forms of elemental carbon that occurs in nature—diamond, graphite, and amorphous carbon—are solids with extremely high melting points and are insoluble in all solvents at ordinary temperatures. The physical properties of the three forms differ widely because of the differences in crystalline structure. In diamond, the hardest material known, each atom is linked to four other atoms in a three-dimensional framework, whereas graphite consists of weakly bonded plane layers of atoms that are arranged in hexagons. Amorphous carbon is characterized by a very low degree of crystallinity. Pure amorphous carbon can be obtained by heating purified sugar at 900° C (1652° F) in the absence of air. A fourth form of naturally occurring carbon is a whole class of fullerenes, the best known of which is Buckminsterfullerene. Diamond, crystal form of pure carbon, hardest of all substances, prized as a precious gemstone. The English name is derived from the Latin word adamas meaning “invincible”. Properties Like graphite and charcoal, which is non-crystalline, diamond is an allotrope of carbon. It is the structure of its crystal lattice and the uniform bonding of the atoms within that together produce its exceptional optical and physical properties. Its scratch hardness is beyond that of all other materials. In fact, only diamond will scratch diamond. Formation Diamonds are often much older than the two host rocks—kimberlite or lamproite— in which they were transported to the surface. Crystallized from pure carbon under great heat and pressure in the Earth’s upper mantle at depths of at least 180 km (112 mi), diamonds may vary in age from between 660 and 3,300 million years (within the Precambrian period).