[Lat., = charcoal], nonmetallic chemical element; symbol C; at. no. 6; at. wt. 12.011; m.p. about 3550 degrees C; graphite sublimes about 3375 degrees C; b.p. 4827 degrees C; sp. gr. 1.8-2.1 (amorphous), 1.9-2.3 (graphite), 3.15-3.53 (diamond); valence +2, =3, =4, or -4. Although carbon makes up only .-32% of the earth's crust, it is very widely distributed and forms a vast number of compounds. There are more carbon compounds than there are compounds of all other elements combined. Carbon exists in the stars; a series of thermonuclear reactions called the carbon cycle (see nucleosynthesis) is a source of energy for some stars. Carbon in the form of diamonds has been found in meteorites. It is found free in nature in at least three distinct forms (see Allotropy). One form, Graphite, is a very soft, dark gray or black, lustrous material with either a hexagonal or rhombohedreal crystalline structure. Diamond, a second crystalline form, is the hardest substance known. In a third form, the so-called amorphous carbon, the element occurs partly free and partly combined with other elements; Charcoal, coal, coke, lampblack, peat, and lignite are some sources of amorphous carbon. A fourth form, "White" carbon, is believed to exist. Carbon has the capacity to act chemically both as a metal and as a nonmetal. It is a constituent of all organic matter. The study of carbon compounds, both natural and synthetic, is called organic chemistry. Plastics, foods, textiles, and many other common substances contain carbon. Hydrocarbon fuels (e.g. natural gas), marsh gas, and the gases resulting from the combustion of fuels (e.g. carbon monoxide and cabon dioxide) are compounds of carbon. With oxygen and a metallic element, carbon forms many important carbonates, such as calcium carbonate (limestone) and sodium carbonate (soda). Certain active metals react with it to make industrially important carbides, such as silicon carbide (known as carborundum), calcium carbide, used for producing acetylene gas, and tungsten carbide, an extremely hard substance used for rock drills and metalworking tools. Coke is used as a fuel in the production of iron. Carbon electrodes are widely used in electrical apparatus. The "lead" of the ordinary pencil is graphite mixed with clay. The successful linking in the 1940s of carbon with silicon has led to the development of a vast number of new substances known collectively as the silicones. All living organisms contain carbon; the human body is about 18% carbon by weight. In green plants carbon dioxide and water are combined to form simple sugars (carbohydrates); light from the sun provides the energy for this process (photosynthesis). The energy from the sun is stored in the chemical bonds of the sugar molecule. Anabolism, the synthesis of complex compounds (such as fats, proteins, and nucleic acids) from simpler substances, involves the utilization of energy stored by photosynthesis. Catabolism is the release of stored energy by the oxidative destruction of organic compounds; water and carbon dioxide are two by-products of catabolism. This continuing synthesis and degradation involving carbon dioxide is known as the biological carbon cycle. Seven isotopes of carbon are known. Carbon-12 was chosen by the IUPAC in 1961 as the basis for Atomic Weights; it is assigned an atomic mass of exactly 12 atomic mass units. Carbon-13 is used as a radioactive tracer. Carbon-14, which has a half-life of 5,730 years, is a naturally occurring isotope that can also be produced in a nuclear reactor. It is used extensively as a research tool in tracer studies; a compound synthesized with carbon-14 is said to be "tagged" and can be traced through a chemical or biochemical reaction. Carbon-14 has been used in the study of such problems as utilization of foods in animal nutrition, catalytic petroleum processes, photosynthesis, and the mechanism of aging in steel. It is also used for determining the age of archaeological specimens (see dating, geologic). Carbon has been known to man in its various forms since ancient times. See Isaac Asimov, The World of Carbon (rev. ed. 1966); P. L. Walker, Jr., and P . A. Thrower, ed., Chemistry and Physics of Carbon (11 vol. 1966-74).
Carbon cycle, in biology, the exchange of carbon between living organisms and the nonliving environment. Living organisms are composed of matter derived from the environment and engage in a continual exchange of matter with their surroundings; as old cells die and their materials return to the environment, new cells are formed of newly incorporated substances. Carbon is the central element in most compounds of which organisms are composed, and it is derived from free cabon dioxide, that is found in air (or, in an aquatic environment, in water). The process of incorporating inorganic molecules into the more complex molecules of living matter is called fixation. Nearly all carbon dioxide fixation is accomplished by means of Photosynthesis, in which green plants form carbohydrates from carbon dioxide and water, using the energy of sunlight to drive the chemical reactions involved. A few microorganisms fix insignificant amounts of carbon dioxide by using other energy sources, such as oxidation of iron. Green plants use carbohydrates to build the other organic molecules that make up their cells, such as cellulose, fats, proteins, and nucleic acids. Some of these compounds require the incorporation of nitrogen (see nitrogen cycle). When carbohydrates are oxidized in cells they release the energy stored in their chemical bonds, and some of that energy is also used by the cell to drive other reactions. In the process of oxidation, or respiration, oxygen from the atmosphere (or from water) is combined with portions of the carbohydrate molecule, producing carbon dioxide and water, the compounds from which the carbohydrates were originally formed. However, not all of the carbon atoms incorporated by the plant can be returned to the atmosphere by its own respiration; some remain fixed in the organic materials that make up its cells. When the plant dies, its tissues are consumed by bacteria and other microorganisms, a process called decay. These microorganisms, which cannot make carbohydrates from carbon dioxide and water, break down the organic molecules of the plant and use them for their own cell-building and energy needs; by their respiration more of the carbon is returned to the atmosphere. Animals, which likewise cannot make their own carbohydrates, feed on plants or on other animals; ultimately their matter and energy are derived from plants. The carbon-containing molecules that an animal derives from other organisms are reorganized to build its own cells or oxidized for energy by respiration, releasing cabon dioxide and water. When the animal dies it too is decayed by microorganisms, resulting in the return of more carbon to the atmosphere. Carbon-containing molecules in wood (or other dry, slow-decaying organic materials) may be oxidized by burning, or combustion, also producing carbon dioxide and water. Under conditions prevailing on earth at certain times, green plants have decayed only partially and have been transformed into fossil fuels--coal, peat, and oil. These materials are made of organic compounds formed by the plants; when burned, they too restore carbon dioxide to the atmosphere.
[The New Columbia Encyclopedia. Harris, William H., and Judith S. Levey, eds. New York and London: Columbia University Press, 1975.]