Carbon is a nonmetallic chemical element found widely in nature. The sixth most abundant element in the universe, carbon plays an essential role in the thermonuclear "burning" of hydrogen in the hotter stars (see \Tcarbon cycle, astronomy\t). On Earth, carbon is found both in native form and in compounds with other elements, making up about 0.2% by weight of the Earth's crust. The element is found in its purest form as \Tdiamond\t and graphite and in less pure form as a constituent of natural coal (see \Tcoal and coal mining\t), \Tcoke\t, and \Tcharcoal\t. Its most abundant compounds are carbon dioxide, which constitutes about 0.05% of the atmosphere and is found dissolved in all natural waters; the \Tcarbonate minerals\t, such as limestone and marble; and the hydrocarbons (see \Thydrocarbon\t), which are principal constituents of coal, \Tpetroleum\t, and \Tnatural gas\t. Carbon is the most versatile element known; 94% of all known compounds (more than 4 million) contain it. Only carbon is capable of combining with other elements in arrangements of sufficient variety and complexity to fulfill the essential functions on which life depends. Certain carbon compounds make up about 18% of the matter in living things (the remainder is mostly water). These compounds function as the blueprints for living cells, as the building blocks from which cells are made, and as the apparatus that builds the cells. Other carbon compounds serve as \Lfuel\ls and are continually replenished by \Tphotosynthesis\t in green plants. The exchange of carbon with the environment ceases when an organism dies, and the amount of the radioactive isotope carbon-14 remaining can be used to determine the age of materials of biological origin (see \Tradiometric age-dating\t). A major segment of the economy of developed nations is devoted to the processing and manufacture of carbon-containing fuels, plastics, chemicals, fabrics, and drugs. The manufacture and use of synthetic, carbon-based compounds have had a profound effect on the standard of living in many countries. CHARACTERISTICS OF THE CARBON ATOM Carbon (symbol C, atomic number 6, atomic weight 12.011) is a member of group \TIva\t of the periodic table along with \Tsilicon\t, \Tgermanium\t, \Ttin\t, and \Tlead\t. It is the lightest and least metallic of these elements. Unlike many other groups in the periodic table, the group \TIva\t elements differ greatly from each other in their chemistry, with carbon being the least representative in its behavior. Its most abundant \Tisotope\t is carbon-12, which makes up 98.89% of naturally occurring carbon. It is used as the international standard for atomic weight, representing exactly 12 daltons (atomic mass units). Carbon-13, constituting 1.11% of natural carbon, is the only other stable isotope. Five radioactive isotopes are known, of which carbon-14 (half-life 5,730 years) is the most stable and useful. Bonds The free carbon atom has two electrons in the 1s shell and four valence electrons in the 2s and 2p shells that are available for bonding (see \Telectron configuration\t). Unlike the metals and many of the nonmetals, the bonding in carbon is generally covalent rather than ionic (see \Tchemical bond\t). One reason for this is that carbon has a low atomic number and therefore holds very tightly to its valence electrons, which are close to the atom's nucleus. Furthermore, a carbon atom must acquire or lose four electrons in order to become a stable ion, an event requiring considerable energy. In covalent bonding, each atom acquires a total of eight electrons by mutually sharing its valence electrons with other atoms. In most carbon compounds an adjacent atom will contribute one to three electrons, which are matched by an equal number from carbon to form a single, double, or triple bond, respectively. The total amount of bonding to a carbon atom--that is, its \Tvalence\t--is normally four. These four bonds may be single or multiple bonds. Carbon forms strong single bonds to itself and to most other elements. It forms very strong double and triple bonds to itself and to nitrogen and oxygen. The endless structural variation and complexity characteristic of the chemistry of carbon ultimately depend on the ability of carbon to bond to itself in long chains. This process, known as catenation, is made possible by the uniform strength of carbon-carbon bonds in all types of situations. There is some evidence for the rare occurrence of pentavalent and hexavalent carbon. Structure Most of the bonding situations for carbon are described spatially in terms of three geometries: tetrahedral, trigonal, and linear. Carbon compounds having four single bonds take on the approximate form of a tetrahedron, having 109.5 deg angles between bonds. The tetrahedron has a high degree of symmetry, which limits the number of \Lisomer\ls that exist for variously substituted carbon compounds (see \Tstereochemistry\t). A carbon atom bearing one double and two single bonds may be described as trigonal--the three groups bonded to carbon take on a symmetrical planar arrangement, with angles of about 120 deg between bonds. A carbon atom with one single and one triple bond, or two double bonds, is held in linear arrangement with the two groups on opposite sides of the carbon atom. More unusual bonding situations also occur. BASIC ELEMENTAL FORMS Pure carbon exists as any of four \Lallotrope\ls, which are different crystal forms of the same element. These allotropes are diamond, graphite, amorphous carbon, and the fullerenes. Diamond Pure diamond is the hardest substance known. Although pure diamond is colorless and transparent, when contaminated with other minerals it may appear in various colors ranging from pastels to opaque black. This crystal is chemically inert but may be induced to burn in air at high temperatures. It is a poor conductor of heat and is an electrical insulator. Until 1955 the only source of diamond was natural deposits of volcanic origin. Since then diamonds have been made artificially from graphite subjected to high pressures and temperatures. Diamonds of gem quality are not made in this way. The properties of diamond are derived from its crystal structure, which is composed entirely of interlocking tetrahedral carbon atoms, each of which is covalently bonded to its four nearest neighbors. A diamond is uniformly bonded throughout and may be thought of as a giant molecule. The exceptional strength of the carbon-carbon bond and the covalently interlocked crystal structure accounts for the hardness and inertness of diamond. Graphite Graphite is a black, lustrous substance that easily crumbles or flakes. It has a slippery feel because of its tendency to cleave from the crystal in thin layers. It is chemically inert, although somewhat less so than a diamond, and is an excellent conductor of both heat and electricity. It occurs as a mineral in nature, usually in somewhat impure form, and can be produced artificially from amorphous carbon. Graphite is composed entirely of planes of trigonal carbon atoms joined in a honeycomb pattern. Each carbon molecule is bonded to three others at 120 deg angles. These planes are arranged in sheets to form three-dimensional crystals. The layers are separated at a distance that represents a nonbonding situation. Because each atom is formally bonded to only three neighboring atoms, the remaining valence electron (one in each atom) is free to circulate within each plane of atoms, contributing to graphite's ability to conduct electricity. One of the main uses for graphite--as a lubricant--results from the characteristic sliding of one layer over another within the crystal. The "lead" in pencils is actually graphite. It is also used as a heat-resistant material; as an electrical conductor and electrode material (in dry cells, for instance); and in nuclear reactors as a neutron moderator. Amorphous Carbon Less well defined than diamond or graphite, amorphous carbon has physical and chemical properties that may vary depending on its method of manufacture and conditions to which it is later subjected. It is a deep black powder that occurs in nature as a component of coal and \Tlignite\t. It may be obtained artificially from almost any organic substance by heating the substance to very high temperatures in the absence of air. In this way coke is produced from coal, and charcoal from wood. Burning organic vapors with insufficient oxygen produces such amorphous forms as carbon black and lampblack. Amorphous carbon is the most reactive form of carbon. It burns relatively easily in air, thereby serving as a fuel, and is attacked by strong oxidants. Amorphous carbon is not a finely divided graphite but appears to have some of the structural features of graphite, such as local regions of sheets and layers. Its atomic structure, however, is much more irregular. The most important uses for carbon black are as a stabilizing filler for rubber and plastics and as a black pigment in inks and paints. Charcoal and coke are used as clean-burning fuels. Certain types of "activated" charcoal are useful as absorbents of gases and of impurities from solutions. Fullerenes In the 1980s scientists determined that another carbon allotrope conjectured for some years as a possibility does indeed occur: carbon atoms linked to form a more or less spherical molecule. A whole family of allotropes, with differing numbers of atoms, is now known to exist. The first to be identified and the most symmetrical of the family, with 60 atoms and 32 sides (20 hexagons and 12 pentagons), was nicknamed "buckyball" and thereafter formally named buckminsterfullerene, because it resembles the geodesic domes of American inventor R. Buckminster \TFuller\t. Spherical carbon molecules as a group are called fullerenes. Buckminsterfullerene is now being produced in marketable quantities for use by scientists. Its occurrence in carbon-containing materials on Earth may yet prove to be widespread, and it is suggested as a fairly common interstellar molecule. Its superconducting properties and its potential for opening new areas of chemistry have made study of the "buckyball" one of the most rapidly expanding areas of research. IMPORTANT CARBON COMPOUNDS Carbon compounds traditionally have been classified as either inorganic or organic. Inorganic carbon compounds are those in which carbon plays a role roughly analogous to its neighboring elements. They include the binary compounds of carbon with other elements that lack carbon-carbon bonds and also salts and complexes of metals that contain simple carbon species. Organic compounds are those reflecting carbon's tendency toward covalent bonding and catenation. They generally contain carbon-carbon and carbon-hydrogen bonds and often have highly complex structures. In organic compounds, functional groups (characteristic arrangements of atoms) are attached to a carbon framework. These functional groups often control the properties and reaction of the compound. Inorganic Compounds Binary compounds of carbon with metals, or metal carbides, have properties ranging from a reactive and saltlike--those of such metals as sodium, magnesium, and aluminum--to an unreactive and metallic nature--as for such transition metals as titanium and niobium. Hafnium carbide has perhaps the highest melting point of any substance, about 3,890 deg C. Carbides of the nonmetals boron and silicon, which are inert and very hard, are used as abrasives. Carbon compounds containing nonmetals are usually gases or low-boiling liquids. Carbon monoxide is a colorless, odorless, and flammable gas that forms during the incomplete combustion of carbon. It is highly toxic to animals because it inhibits the transport of oxygen in the blood by hemoglobin. Because it is a good reducing agent at high temperatures, carbon monoxide is industrially used in the production of metals and organic compounds. Carbon dioxide is a colorless, nearly odorless gas that is formed by combustion of carbon. It is a product of respiration in most living organisms and is used by plants as a source of carbon. Frozen carbon dioxide, known as dry ice, is used as a refrigerant. Carbon disulfide is a colorless, extremely flammable liquid that is used primarily as a solvent. Of the halides of carbon, \Tcarbon tetrachloride\t is the best known. It is a colorless, fairly inert, and toxic liquid that serves as an excellent solvent for fats and oils. \Lfluorocarbon\ls, such as Freon 12, are used as refrigerants. The reaction of carbon monoxide with halogens yields carbonyl halides, including the chloride known as phosgene. Principal inorganic compounds containing carbon and nitrogen are the poisonous gases hydrogen \Tcyanide\t and cyanogen. A variety of metal salts containing the anions cyanide, cyanamide, cyanate, fulminate, and thiocyanate are also important chemicals that are used industrially. Organic Compounds The simplest organic compounds consist only of carbon and hydrogen--the hydrocarbons. Those containing only single bonds are called saturated hydrocarbons or \Lalkane\ls and are composed of chains, rings, or 3-dimensional frameworks of carbon atoms. Those containing up to 4 carbons are gases, and those containing up to about 20 carbons are liquids. Alkanes having higher molecular weights are solids. Unsaturated hydrocarbons are those containing double bonds (\Lalkene\ls) and triple bonds (\Lalkyne\ls). They are more reactive than saturated hydrocarbons but have similar physical properties. Hydrocarbons that are based on a stable six-membered ring of carbons with three double bonds are referred to as \Taromatic compounds\t (this has no relation to odor) and have special types of reactivity. \Tbenzene\t is the simplest molecule of this class. More complex organic molecules are derived from the hydrocarbons by replacing one or more hydrogens with other atoms or functional groups. One major class is the halides. Other classes are \Talcohol\t, \Tether\t, \Tketone\t, \Taldehyde\t, \Tcarboxylic acid\t, \Tamine\t, \Tnitrile\t, and certain organometals. Biochemical Substances Living systems contain many complex molecules that have specific functions. These biochemical substances may be classified according to seven major structural types. The acetogenins include many useful pigments and antibiotics. Nitrogen-containing alkaloids (see \Talkaloid\t) include many major types of drugs. Carbohydrates (see \Tcarbohydrate\t) include sugars, starches, and cellulose and are primary products of photosynthesis. Lipids (see \Tlipid\t) or fats are used for energy storage and as components of cell membranes. Nucleic acids store and transmit genetic information (see \Tgene\t). Peptides (see \Tpeptide\t) and proteins serve as enzymes, hormones, sense receptors, structural and mechanical components, transport proteins, energy-transfer proteins, and gene regulators (see \Tproteins and protein synthesis\t). Terpenes (see \Tterpene\t) and steroids (see \Tsteroid\t) include many hormones and pheromones and often serve as flavor and odor components. SOCIOECONOMIC ROLE OF CARBON Carbon and its compounds have assumed a position of major influence on the economy, especially in the more industrialized nations. \Tsynthetic fibers\t and \Tplastics\t, which are ultimately derived from petroleum-based chemicals (\Tpetrochemicals\t), have largely supplanted natural substances. These materials serve as the basis for numerous technological advances, including those in such fields as the electronics and transportation industries. The development of synthetic \Ldrug\ls has contributed enormously both to relieving suffering and to helping to understand the molecular machinery of life. The carbon-based fuels, which have provided most of the world's energy needs since 1900, have been increasingly less available and more expensive. Methods to process coal and living matter to produce carbon-based synthetic fuels also means risking a further increase in already serious levels of air pollution (see \Tpollution, environmental\t) by the combustion of hydrocarbons, and of \Tacid rain\t, a result of burning sulfur containing fuels. David C. Roberts Bibliography: Baggott, Jim, "Great Balls of Carbon," New Scientist, July 6, 1991; Baum, R.M., "Studies Support Spherical Structure, Aromaticity of C60 Carbon Clusters," Chemical & Engineering News, Aug. 29, 1988; Bonds, R. L., Porous Carbon Solids (1967); Edwards, Ian A.S., et al., Introduction to Carbon Science (1989); Ermolenko, I.N., Chemically Modified Carbon Fibers and Their Applications (1990); Kinoshita, Kim, Carbon (1988); Mattson, J. S., and Mark, H. B., Jr., Activated Carbon (1971); Urry, Grant, Elementary Equilibrium Chemistry of Carbon (1989); Walker, P. L., and Thrower, P. A., eds., Chemistry and Physics of Carbon, vols. 1-22 (1966-89).