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carbon[Lat.,=charcoal], nonmetallic chemical element; symbol C; at. no. 6; interval in which at. wt. ranges 12.0096–12.0116; m.p. about 3,550°C;; graphite sublimes about 3,375°C;; b.p. 4,827°C;; sp. gr. 1.8–2.1 (amorphous), 1.9–2.3 (graphite), 3.15–3.53 (diamond); valence +2, +3, +4, or −4.
Properties and Isotopes
Carbon is found free in nature in at least four distinct forms (see allotropyallotropy
[Gr.,=other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes.
..... Click the link for more information. ). One form, graphitegraphite
, an allotropic form of carbon, known also as plumbago and black lead. It is dark gray or black, crystalline (often in the form of slippery scales), greasy, and soft, with a metallic luster.
..... Click the link for more information. , is a very soft, dark gray or black, lustrous material with either a hexagonal or rhombohedral crystalline structure. Diamonddiamond,
mineral, one of two crystalline forms of the element carbon (see allotropy), the hardest natural substance known, used as a gem and in industry. Properties
..... Click the link for more information. , 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; charcoalcharcoal,
substance obtained by partial burning or carbonization (destructive distillation) of organic material. It is largely pure carbon. The entry of air during the carbonization process is controlled so that the organic material does not turn to ash, as in a conventional
..... Click the link for more information. , coalcoal,
fuel substance of plant origin, largely or almost entirely composed of carbon with varying amounts of mineral matter. Types
There is a complete series of carbonaceous fuels, which differ from each other in the relative amounts of moisture, volatile matter,
..... Click the link for more information. , cokecoke,
substance obtained by the destructive distillation of bituminous coal. Coke bears the same relation to coal as does charcoal to wood. A hard, gray, massive, porous fuel, coke is the solid residue remaining after bituminous coal is heated to a high temperature out of
..... Click the link for more information. , lampblack, peatpeat,
soil material consisting of partially decomposed organic matter, usually found in swamps and bogs in various parts of the temperate zone. It is formed by the slow decay of successive layers of aquatic and semiaquatic plants, e.g., sedges, reeds, rushes, and mosses.
..... Click the link for more information. , and lignitelignite
or brown coal,
carbonaceous fuel intermediate between coal and peat, brown or yellowish in color and woody in texture. It contains more moisture than coal and tends to dry and crumble when exposed to the air; the flame is long and smoky and the heating power
..... Click the link for more information. are some sources of amorphous carbon. A fourth form contains the fullerenesfullerene,
any of a class of carbon molecules in which the carbon atoms are arranged into 12 pentagonal faces and 2 or more hexagonal faces to form a hollow sphere, cylinder, or similar figure.
..... Click the link for more information. , stable molecules consisting of carbon atoms that arrange themselves into 12 pentagonal faces and any number greater than 1 of hexagonal faces. The most prominent of the fullerenes is buckminsterfullerenebuckminsterfullerene
C60, hollow cage carbon molecule named for R. Buckminster Fuller because of the resemblance of its molecular structure to his geodesic domes.
..... Click the link for more information. , a spheroidal molecule, resembling a soccer ball, consisting of 60 carbon atoms. A fifth 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.
Carbon has 13 known isotopes, which have from 2 to 14 neutrons in the nucleus and mass numbers from 8 to 20. Carbon-12 was chosen by IUPAC in 1961 as the basis for atomic weightsatomic weight,
mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent.
..... Click the link for more information. ; it is assigned an atomic mass of exactly 12 atomic mass units. Carbon-13 absorbs radio waves and is used in nuclear magnetic resonance spectrometry to study organic compounds. 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 datingdating,
the determination of the age of an object, of a natural phenomenon, or of a series of events. There are two basic types of dating methods, relative and absolute.
..... Click the link for more information. ).
There are more carbon compounds than there are compounds of all other elements combined. The study of carbon compounds, both natural and synthetic, is called organic chemistry. Plasticsplastic,
any organic material with the ability to flow into a desired shape when heat and pressure are applied to it and to retain the shape when they are withdrawn. Composition and Types of Plastic
..... Click the link for more information. , foods, textilestextiles,
all fabrics made by weaving, felting, knitting, braiding, or netting, from the various textile fibers (see fiber). Types of Textiles
Textiles are classified according to their component fibers into silk, wool, linen, cotton, such synthetic fibers as
..... Click the link for more information. , and many other common substances contain carbon. Hydrocarbonhydrocarbon
, any organic compound composed solely of the elements hydrogen and carbon. The hydrocarbons differ both in the total number of carbon and hydrogen atoms in their molecules and in the proportion of hydrogen to carbon.
..... Click the link for more information. fuels (e.g., natural gas), marsh gas, and the gases resulting from the combustion of fuels (e.g., carbon monoxide and carbon 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 (an abrasive known as carborundum), calcium carbide, used for producing acetylene gas, and tungsten carbide, an extremely hard substance used for rock drills and metalworking tools.
Natural Occurrence and Uses
Carbon has been known to humans in its various forms since ancient times. Although carbon makes up only .032% of the earth's crust, it is very widely distributed and forms a vast number of compounds. Carbon exists in the stars; a series of thermonuclear reactions called the carbon cycle (see nucleosynthesisnucleosynthesis
in astronomy, production of all the chemical elements from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleus; nuclear energy).
..... Click the link for more information. ) is a source of energy for some stars. Carbon in the form of diamonds has been found in meteorites. 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 siliconessilicone,
polymer in which atoms of silicon and oxygen alternate in a chain; various organic radicals, such as the methyl group, CH3, are bound to the silicon atoms.
..... Click the link for more information. .
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 (carbohydratescarbohydrate,
any member of a large class of chemical compounds that includes sugars, starches, cellulose, and related compounds. These compounds are produced naturally by green plants from carbon dioxide and water (see photosynthesis).
..... Click the link for more information. ); light from the sun provides the energy for this process (photosynthesisphotosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
..... Click the link for more information. ). The energy from the sun is stored in the chemical bonds of the sugar molecule. Anabolism, the synthesis of complex compounds (such as fatsfats and oils,
group of organic substances that form an important part of the diet and also are useful in many industries. The fats are usually solid, the oils generally liquid at ordinary room temperatures.
..... Click the link for more information. , proteinsprotein,
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
..... Click the link for more information. , and nucleic acidsnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
..... Click the link for more information. ) 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 byproducts of catabolism. This continuing synthesis and degradation involving carbon dioxide is known as the biological carbon cyclecarbon cycle,
in biology, the exchange of carbon between living organisms and the nonliving environment. Inorganic carbon dioxide in the atmosphere is converted by plants into simple carbohydrates, which are then used to produce more complex substances.
..... Click the link for more information. .
See P. L. Walker, Jr., and P. A. Thrower, ed., Chemistry and Physics of Carbon (11 vol., 1966–74); H. O. Pierson, Handbook of Carbon, Graphite, Diamond, and Fullerenes: Properties, Processing, and Applications (1993).
C, a chemical element of group IV of Mendeleev’s periodic system. Atomic number, 6; atomic weight, 12.011. The two known stable isotopes of carbon are 12C (98.892 percent) and13C (1.108 percent). The most important radioisotope is 14C, with a half-life of 5.6 × 103 years. Small amounts of 14C (approximately 2 × 10–10 percent by weight) are constantly being formed in the upper layers of the atmosphere by the action of cosmic neutron radiation on 14N. The age of biological remains is determined by the specific activity of 14C. This isotope is also commonly used as an isotope tracer.
Historical survey. Carbon has been known since antiquity. Charcoal was used to reduce metals from ores, and diamonds were valued as gems. The use of graphite in the production of crucibles and pencils began much later.
In 1778, K. Scheele discovered that carbon dioxide is liberated when graphite is heated with saltpeter, the same result as that obtained by heating coal with saltpeter. The chemical composition of diamond was established by A. Lavoisier, who in 1772 studied the combustion of diamond in air, and by S. Tennant, who in 1797 showed that equal amounts of diamond and coal yield, upon oxidation, equal amounts of carbon dioxide. Carbon was recognized as a chemical element in 1789 by Lavoisier. Its name comes from the Latin word carbo, meaning coal.
Distribution in nature. The average content of carbon in the earth’s crust is 2.3 × 10–2 percent by weight (1 × 10–2 percent in ultrabasic rock, 1 × 10–2 percent in basic rock, 2 × 10–2 percent in intermediate rock, and 3 × 10–2 percent in acidic rock). Carbon accumulates in the upper portion of the earth’s crust (biosphere). The content of carbon is 18 percent in living matter, 50 percent in wood, 80 percent in coal, 85 percent in petroleum, and 96 percent in anthracite. A significant portion of the carbon in the lithosphere is concentrated in limestones and dolomites.
There are 112 minerals of carbon as such. The number of organic compounds of carbon, namely, hydrocarbons and hydrocarbon derivatives, is exceedingly large.
The accumulation of carbon in the earth’s crust results in the accumulation of many other elements that are sorbed by organic matter and precipitated in the form of, for example, insoluble carbonates. Carbon dioxide and carbonic acid have important geochemical roles in the earth’s crust. Enormous amounts of carbon dioxide are liberated as a result of volcanic activity; historically, this activity was the principal source of carbon in the biosphere.
Mankind has extracted carbon from the earth in quantities that are extremely large in relation to the element’s average content in the crust. The carbon is extracted in the form of coal, petroleum, and natural gas, substances that are major sources of energy.
The carbon cycle has enormous geochemical significance. The biological role of the element is discussed below.
Carbon is also widely distributed in space; it is the sun’s fourth most abundant element, after hydrogen, helium, and oxygen.
Physical and chemical properties. The four known crystalline modifications of carbon are graphite, diamond, artificial graphite, and lonsdaleite. Graphite is a gray-black, opaque, scaly, very soft substance that has a metallic luster and is greasy to the touch. It consists of hexagonal crystals, with a = 2.462 angstroms (Å) and c = 6.701 Å. At room temperature and normal pressure (0.1 meganewton per sq m [MN/m2], or 1 kilogram-force per sq cm [kgf/cm2]), graphite is thermodynamically stable.
Diamond is a very hard crystalline substance. Its crystals have a cubic face-centered lattice, with a = 3.560 Å. At room temperature and normal pressure, diamond is metastable. Diamond converts into graphite to a marked extent at temperatures above 1400°C in a vacuum or an inert atmosphere. Graphite sublimes at a temperature of approximately 3700°C at atmospheric pressure. Liquid carbon can be obtained at pressures greater than 10.5 MN/m2 (105 kgf/cm2) and temperatures above 3700°C. Solid carbon can also have a disordered structure. This amorphous form, which includes coke, carbon black, and charcoal, is not an independent modification of carbon, and its structure is basically that of finely crystalline graphite. Heating several varieties of amorphous carbon above 1500°-1600°C without access to the air induces conversion into graphite. The physical properties of amorphous carbon depend to a great extent on the degree of dispersion of the particles and on the presence of impurities. The density, heat capacity, heat conductivity, and electric conductivity of amorphous carbon are always higher than those of graphite. Artificial graphite is a finely crystalline black powder with a density of 1.9–2 g/cm3; it is made up of long chains of carbon atoms arranged in parallel fashion. Lonsdaleite was discovered in meteorites and can be obtained artificially; its structure and properties have not yet been finally established.
The configuration of the outer electron subshells of the carbon atom is 2s22p2. Carbon characteristically forms four covalent bonds as a result of the excitation of the outer shell to the 2sp3 state. Thus, carbon is equally capable of accepting and donating electrons. The chemical bonds of carbon may involve sp3, sp2, or sp hybrid orbitals, which correspond to coordination numbers of 4, 3, and 2. The number of valence electrons of carbon and the number of valence orbitals are identical, which is one of the reasons for the stability of the bonds between carbon atoms.
The unique capacity of carbon atoms to form bonds with one another resulting in long stable chains and rings accounts for the enormous number and great variety of carbon compounds, the study of which is the province of organic chemistry.
Oxidation states of —4, +2 and +4 are observed in carbon compounds. The element’s atomic radius is 0.77 A, and covalent radii of 0.77 Å, 0.67 Å, and 0.60 Å correspond to, respectively, single, double, and triple bonds. The ionic radius of C4- is 2.60 Å, while that of C4+ is 0.20 Å. Under ordinary conditions, carbon is chemically inert, but at high temperatures it combines with many elements and displays strong reducing properties. The chemical reactivity of carbon decreases in the order amorphous carbon > graphite > diamond; the reactions of these three forms of carbon with atmospheric oxygen (combustion) occur, respectively, at temperatures above 300°-500°C, 600°-700°C, and 850°-1000°C and result in the formation of carbon dioxide (CO2) and carbon monoxide (CO).
Carbon dioxide dissolves in water to form carbonic acid. In 1906, O. Diels obtained carbon suboxide (C302). All forms of carbon are stable toward alkalies and acids; they are slowly oxidized only by very strong oxidizing agents, such as chromium mixtures or mixtures of concentrated HN03 and KC103. Amorphous carbon reacts with fluorine at room temperature, while graphite and diamond react with this element only upon heating. The direct combination of carbon with chlorine occurs in an electric arc. Since carbon does not react with bromine and iodine, many carbon halides are obtained indirectly. Carbonyl chloride (COCl2, phosgene) is the best known of the carbon oxyhalides, which have the general formula COX2, × being the halogen. While hydrogen does not react with diamond, it does react with graphite and amorphous carbon at high temperatures in the presence of catalysts (Ni, Pt); the main product is methane (CH4) at 600°-1000°C and acetylene (C2H2) at 1500°-2000°C. Other hydrocarbons, for example, ethane (C2H6) and benzene (C6H6), may also be present in the products.
Sulfur begins to react with amorphous carbon and graphite at 700°-800°C; with diamond, however, the temperature must be 900°-1000°C. In all cases, the product is carbon disulfide (CS2). Other sulfur-containing carbon compounds, such as carbon monosulfide (CS), carbon thiosuboxide (C3S2), carbonyl sulfide (COS), and thiophosgene (CSC12), are obtained indirectly. Thio-carbonates, the salts of the weak acid thiocarbonic acid, are formed from the reaction of CS2 with metal sulfides. The reaction of carbon with nitrogen to yield cyanogen [(CN)2] accompanies an electric discharge between carbon electrodes in a nitrogen atmosphere. Of the nitrogen-containing carbon compounds, hydrogen cyanide (HCN) and its numerous derivatives, namely, cyanides, cyanogen halides, and nitriles, have the greatest practical importance.
At temperatures above 1000°C, carbon reacts with many metals to yield carbides. Upon heating, all forms of carbon reduce metal oxides, forming free metals (Zn, Cd, Cu, Pb) or carbides (CaC2, Mo2C, WC, TaC). Carbon reacts at temperatures above 600°-800°C with steam and carbon dioxide. A distinguishing feature of graphite is the capacity to react with alkali metals and halides upon moderate heating (to 300°-400oC) to yield inclusion compounds of the type C8M, C24M, or C8X, where × is a halogen and M is a metal. Inclusion compounds of graphite and HN03, H2SO4, and FeCl3 are known, for example, graphite bisulfate (C24SO4H2). All forms of carbon are insoluble in ordinary inorganic and organic solvents but soluble in some molten metals, among them, Fe, Ni, and Co.
Economic importance. The importance of carbon in the economy derives from the element’s presence in fossil fuels, the primary source for more than 90 percent of the energy consumed by the world. Fossil fuels will retain their dominant position in the next few decades despite the rapid development of nuclear power. Only approximately 10 percent of the fossil fuels extracted is used as a raw material in industrial organic synthesis, petrochemical synthesis, and the production of plastics.
B. A. POPOVKIN
In organisms. Carbon is the most important biological element, providing the basis for life on the earth and serving as a structural unit for the enormous number of organic compounds that make up an organism and participate in the organism’s vital activities. Examples are provided by biopolymers and by the many low-molecular-weight biologically active substances, such as vitamins, hormones, and mediators. A significant part of the energy required by an organism is produced by the oxidation of carbon in the cells. The origin of life on the earth is considered by modern science to have been a complex process of evolution of carbon compounds.
The unique role of carbon in living things derives from the element’s properties, the aggregate of which is not possessed by any other element of the periodic system. Strong chemical bonds are formed between carbon atoms, as well as between carbon and other elements, but these bonds (single, double, triple) may be cleaved under relatively mild physiological conditions. The capacity of carbon to form four equivalent valence bonds with other carbon atoms permits the construction of different types of carbon skeletons, including linear, branched-chain, and cyclic structures. It is significant that just three elements, namely, carbon, oxygen, and hydrogen, constitute 98 percent of the total mass of living organisms. This fact makes possible a certain economy in living things; with the practically limitless structural diversity of carbon compounds, the existence of just a few types of chemical bonds permits a significant reduction in the number of enzymes required for the breakdown and synthesis of organic compounds. The special features of the carbon atom’s structure account for the various types of isomerism in organic compounds. (The tendency toward optical isomerism proved to be decisive in the biochemical evolution of amino acids, carbohydrates, and certain alkaloids.)
According to the generally accepted hypothesis of A. I. Opa-rin, the first organic compounds on earth were abiological. The sources of carbon were the methane (CH4) and hydrogen cyanide (HCN) present in the earth’s primordial atmosphere. With the appearance of life, the only source of inorganic carbon, through which all the organic matter in the biosphere was formed, was carbon dioxide (CO2); this compound was present in the atmosphere and was dissolved in the earth’s waters in HCO3– form.
The most powerful mechanism for the assimilation of carbon (in the form of CO2) is photosynthesis, which is everywhere carried out by green plants. (Approximately 100 billion tons of CO2are assimilated annually.) Another method of assimilating CO2, which evolved earlier, is chemosynthesis. Here, chemosynthetic bacteria use, rather than radiant energy from the sun, the energy derived from the oxidation of inorganic compounds. The carbon contained in the food of most animals is in the form of organic compounds. Depending on the method of assimilating organic compounds, a distinction is made between autotrophic and heterotrophic organisms. The possibility of using those microorganisms whose only source of carbon is petroleum hydrocarbons for the biosynthesis of protein and other nutrients is presently the focus of much attention in modern science and technology.
The content of carbon in living organisms, as a percentage of dry mass, is 34.5–40 percent in aquatic plants and animals, 45.4–46.5 percent in terrestrial plants and animals, and 54 percent in bacteria. The vital activities of an organism, especially tissue respiration, are accompanied by the oxidative decomposition of organic compounds and the liberation of CO2 into the external medium. Carbon is also liberated in more complex metabolic end products. As plants and animals die, a portion of the carbon is reconverted into CO2 as a result of decay processes carried out by microorganisms. Thus, a carbon cycle can be observed in nature. A significant portion of carbon is mineralized, forming deposits of, for example, coal, petroleum, and limestone. While CO2functions mainly as a source of carbon, it also, when dissolved in water and biological fluids, helps to maintain the optimal level of acidity for an organism’s vital activities. In the form of CaCO3, carbon forms the outer skeleton of many invertebrates, for example, the shells of mollusks; it is also present in corals and the eggshells of birds. Such carbon compounds as HCN, CO, and CC14 predominated in the earth’s atmosphere in the period before the appearance of life; subsequently, in the course of biological evolution, these compounds become strong antimetabolites.
In addition to the stable isotopes of carbon, radioactive l4C is widespread in nature. Its content in humans is approximately 1 microcurie. Many great advances in the study of metabolism and of the carbon cycle in nature have been linked to the use of carbon isotopes in biological and medical research. Thus, radiocarbon marking has been used to prove the possibility of the fixation of H14CO~3 by plants and animal tissues, establish the sequence of reactions in photosynthesis, study the metabolism of amino acids, and trace the biosynthetic pathways of biologically active compounds. The use of 14C has facilitated advances in molecular biology in the study of the mechanisms of protein biosynthesis and the transfer of genetic information. The evaluation of the age of carbon-containing organic remains through the determination of the specific activity of, 4C is particularly useful in paleontology and archaeology.
N. N. CHERNOV
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