Carbon
From Encyclopædia
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 CARBON CYCLE, ASTRONOMY). 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 DIAMOND and graphite and in less pure form as a constituent of natural coal (see
coal and coal mining), COKE, and CHARCOAL. Its most abundant compounds are carbon dioxide, which constitutes about 0.05% of the
atmosphere and is found dissolved in all natural waters; the CARBONATE MINERALS, such as
limestone and marble; and the hydrocarbons (see HYDROCARBON), which are principal constituents of coal,
petroleum, and
natural gas.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 FUELS and are continually replenished by PHOTOSYNTHESIS 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 RADIOMETRIC AGE-DATING).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 ATOMCarbon (symbol C,
atomic number 6,
atomic weight 12.011) is a member of group IVA of the
periodic table along with
silicon, GERMANIUM, TIN, and
lead. It is the lightest and least metallic of these elements. Unlike many other groups in the
periodic table, the group IVA elements differ greatly from each other in their chemistry, with carbon being the least representative in its behavior.Its most abundant ISOTOPE 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.BondsThe 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
electron CONFIGURATION). Unlike the
metals and many of the nonmetals, the bonding in carbon is generally covalent rather than ionic (see CHEMICAL BOND). 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 VALENCE--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.StructureMost 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 ISOMERS that exist for variously substituted carbon compounds (see
STEREOCHEMISTRY).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 FORMSPure carbon exists as any of four ALLOTROPES,?which are different
crystal forms of the same element. These allotropes are diamond, graphite, amorphous carbon, and the fullerenes.DiamondPure 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.GraphiteGraphite 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 CarbonLess 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 LIGNITE. 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.FullerenesIn 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 an? 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 FULLER. Spherical carbon
molecules as a group are called fullerenes.
