Carbon

CARBON, C, a tetravalent element with an atomic weight of 12.000 (the isotope of carbon has an atomic weight of 13). It begins the IV group of elements. Its atomic number is 6, i.e., it has 6 electrons, 4 of which, being in the outer sphere, determine the ease with which carbon forms compounds with elements of both the I and VII groups. For example, compounds with electropositive hydrogen—methane CH₄—and with the electronegative element chlorine—carbon tetrachloride CCl₄—are stable. Pure carbon is found in nature in two allotropic modifications: diamond and graphite. So-called amorphous carbon consists of the smallest submicroscopic crystals, the atomic structure of which does not differ from graphite. Amorphous carbon usually also contains impurities of other elements, which, along with its finely divided state, determines the difference in physical properties between coal and graphite. The transition of one allotropic form to another can be carried out in the direction diamond -> graphite by increasing the temperature. The reverse transition from the stable form of graphite to the less stable form of diamond is significantly more difficult. By applying high temperature and pressure, it has been possible to obtain diamond from graphite only in insignificant quantities and in a finely divided state. Physical properties are sharply different for different modifications, as can be seen from the table provided: Properties: Specific gravity, Hardness on the Mohs scale, Crystalline form, Color, Atomic volume, Specific electrical resistance at 0° (ohm), Refractive index, Atomic heat capacity (cal/g-atom), Ignition temperature in pure O₂; Coal: 1.81, amorphous (microcrystalline), black, 3.5 (average), variable, 2.0-3.1, 300-400°. X-ray examination has made it possible to establish the arrangement of atoms in various modifications of carbon. In diamond, carbon atoms are arranged in such a way that each atom lies in the center of a tetrahedron formed by other carbon atoms located at a distance of 1.54 Å from each other. Carbon is arranged similarly in organic compounds of the fatty series. In graphite, carbon atoms are arranged in layers, forming a kind of grid with hexagonal cells. The side of such a hexagon is equal to 1.42-1.45 Å. Carbon atoms are arranged similarly in six-membered polynuclear cycles of aromatic compounds. Carbon does not melt at any temperature and does not dissolve in ordinary solvents (it dissolves in iron at high temperatures). When heated in air, it burns into carbon monoxide CO or carbon dioxide CO₂. Carbon compounds are extremely common in nature in the form of calcium and magnesium carbonates. Carbon is a component of rocks (limestone, dolomite). Carbon forms the main constituent part of such mineral resources as coal, anthracite, peat, oil, and asphalt. Carbon lies at the basis of organic substances, such as proteins, fats, carbohydrates, alcohols, phenols, etc. Regarding the carbon cycle, see Cycle of substances.

A. Kuzin.