Carbon
- Nonmetallic chemical element, chemical symbol C, atomic number 6.
- The usual stable isotope is carbon-12; carbon-13, another stable isotope, makes up 1% of natural carbon.
- Carbon-14 is the most stable and best known of five radioactive isotopes (see radioactivity);
- its half-life of approximately 5,730 years makes it useful in carbon-14 dating and radiolabeling of research compounds.
- Carbon occurs in four known allotropes: Diamond, graphite, carbon black (amorphous carbon including coal, coke, and charcoal), and hollow cage molecules called fullerenes.
- Carbon forms more compounds than all other elements combined; several million carbon compounds are known.
- Each carbon atom forms four bonds (four single bonds, two single and one double bond, two double bonds, or one single and one triple bond) with up to four other atoms.
- Multitudes of chain, branched, ring, and three-dimensional structures can occur.
- The study of these carbon compounds and their properties and reactions is organic chemistry (see organic compound). With hydrogen, oxygen, nitrogen, and a few other elements whose small amounts belie their important roles, carbon forms the compounds that make up all living things: proteins, carbohydrates, lipids, and nucleic acids.
- Biochemistry is the study of how those compounds are synthesized and broken down and how they associate with each other in living organisms.
- Organisms consume carbon and return it to the environment in the carbon cycle. Carbon dioxide, produced when carbon is burned and from biological processes, makes up about 0.03% of the air, and carbon occurs in Earth's crust as carbonate rocks and the hydrocarbons in coal, petroleum, and natural gas. The oceans contain large amounts of dissolved carbon dioxide and carbonates.
- Half Life
Interval of time required for one-half of the atomic nuclei of a radioactive sample to decay (change spontaneously into other nuclear species by emitting particles and energy), or the time required for the number of disintegrations per second of a radioactive material to decrease by one-half.
Half-lives are characteristic properties of the various unstable atomic nuclei and the particular way in which they decay. Alpha decay and beta decay are generally slower processes than gamma decay.
Carbon-14 dating
radiocarbon dating
Method of determining the age of once-living material, developed by U.S. physicist Willard Libby in 1947.
It depends on the decay of the radioactive isotope carbon-14 (radiocarbon) to nitrogen.
All living plants and animals continually take in carbon: green plants absorb it in the form of carbon dioxide from the atmosphere, and it is passed to animals through the food chain.
Some of this carbon is radioactive carbon-14, which slowly decays to the stable isotope nitrogen-14.
When an organism dies it stops taking in carbon, so the amount of carbon-14 in its tissues steadily decreases. Because carbon-14 decays at a constant rate, the time since an organism died can be estimated by measuring the amount of radiocarbon in its remains. T
he method is a useful technique for dating fossils and archaeological specimens from 500 to 50,000 years old and is widely used by geologists, anthropologists, and archaeologists.
Carbon Allotropes
Any of two or more forms of the same chemical element.
They may have different arrangements of atoms in crystals of the solid—for example, graphite and diamond for carbon—or different numbers of atoms in their molecules—for example, ordinary oxygen (O2) and ozone (O3). Other elements that have allotropes include tin, sulfur, antimony, arsenic, selenium, and phosphorus.
Diamond
Mineral composed of pure carbon, the hardest naturally occurring substance known and a valuable gemstone.
Diamonds are formed deep in the Earth by tremendous pressures and temperatures over long periods of time.
In the crystal structure of diamond, each carbon atom is linked to four other, equidistant, carbon atoms. This tight crystal structure results in properties that are very different from those of graphite, the other common form of pure carbon. Diamonds vary from colourless to black and may be transparent, translucent, or opaque. Most gem diamonds are transparent and colourless or nearly so. Colourless or pale blue stones are most valued, but most gem diamonds are tinged with yellow. Because of their extreme hardness, diamonds have important industrial applications. Most industrial diamonds are gray or brown and are translucent or opaque. In the symbolism of gemstones, the diamond represents steadfast love and is the birthstone for April.
Graphite
or plumbago black lead
Mineral allotrope of carbon.
It is dark gray to black, opaque, and very soft. Its layered structure, with rings of six atoms arranged in widely spaced parallel sheets, gives it its slippery quality. It occurs in nature and is used (mixed with clay) as the “lead” in pencils. It is also used in lubricants, crucibles, polishes, arc lamps, batteries, brushes for electric motors, and nuclear reactor cores.
Coal
Solid, usually black but sometimes brown, carbon-rich material that occurs in stratified sedimentary deposits.
One of the most important fossil fuels, it is found in many parts of the world. Coal is formed by heat and pressure over millions of years on vegetation deposited in ancient shallow swamps (see peat). It varies in density, porosity, hardness, and reflectivity. The major types are lignite, subbituminous, bituminous, and anthracite. Coal has long been used as fuel, for power generation, for the production of coke, and as a source of various compounds used in synthesizing dyes, solvents, and drugs. The search for alternative energy sources has periodically revived interest in the conversion of coal into liquid fuels; technologies for coal liquefaction have been known since early in the 20th century.
Coke
Solid residue remaining after certain types of coals are heated to a high temperature out of contact with air until substantially all components that easily vaporize have been driven off.
The residue is chiefly carbon, with minor amounts of hydrogen, nitrogen, sulfur, and oxygen.
Also present in coke is the mineral matter in the original coal, chemically altered and decomposed. The gradual exhaustion of timber in England had led first to prohibitions on cutting of wood for charcoal and eventually to the introduction of coke. Thereafter the iron industry expanded rapidly and Britain became the world's greatest iron producer (see Abraham Darby). The crucible process (1740) resulted in the first reliable steel made by a melting process. Oven coke (about 1.5–4 in., or 40–100 mm, in size) is used in blast furnaces to make iron. Smaller quantities of coke are used in other metallurgical processes (see metallurgy), such as the manufacture of certain alloys. Large, strong coke, known as foundry coke, is used in smelting. Smaller sizes of coke (0.6–1.2 in., or 15–30 mm) are used to heat buildings.
Charcol
Impure form of carbon, obtained as a residue when material containing carbon is partially burned or heated with limited access to air.
Coke, carbon black, and soot are forms of charcoal; other forms are named for their source material, such as wood, blood, or bone. Largely replaced by coke in blast furnaces and by natural gas as a raw material, charcoal is still used to make black gunpowder and in case-hardening metals. Activated charcoal is a finely powdered or highly porous form whose surface area is hundreds or thousands of square meters per gram. It has many uses as an adsorbent (see adsorption), including for poison treatment, and as a catalyst or catalyst carrier.
fullerene(buckminster fullerene)
Allotropes of carbon first identified in 1985, they have varying (but even) numbers of atoms bonded into structures having 12 pentagonal and 2 or more hexagonal faces. Fullerenes comprising dozens to hundreds of carbon atoms have been prepared. The best known and most stable fullerene, buckminsterfullerene (C60, nicknamed buckyball), has 60 carbon atoms arranged in a pattern like that on a standard soccer ball. It is named for R. Buckminster Fuller, whose geodesic dome designs its structure resembles. Chemists have made fullerene derivatives (e.g., with attached hydrogen or halogen atoms or organic groups; see functional group) and have prepared doped fullerenes (e.g., with alkali metal atoms such as potassium; see dopant) that show superconductivity at relatively high temperatures. One or more metal or noble-gas atoms can be trapped in the molecule's hollow interior, resulting in unique complexes called endohedral fullerenes.
Carbon Cycle
Circulation through nature of carbon in the form of the simple element and its compounds.
The source of carbon in living things is carbon dioxide (CO2) from air or dissolved in water. Algae and green plants (producers) use CO2 in photosynthesis to make carbohydrates, which in turn are used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them. The carbon may pass through several levels of herbivores and carnivores (consumers). Animals and, at night, plants return the CO2 to the atmosphere as a by-product of respiration. The carbon in animal wastes and in the bodies of organisms is released as CO2 in a series of steps by decay organisms (decomposers), chiefly bacteria and fungi (see fungus). Some organic carbon (the remains of organisms) has accumulated in Earth's crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehistoric times, is being returned in vast quantities as CO2 via industrial and agricultural processes, some accumulating in the oceans as dissolved carbonates and some staying in the atmosphere (see greenhouse effect).
Carbon Dioxide
Inorganic compound, a colourless gas with a faint, sharp odour and a sour taste when dissolved in water, chemical formula CO2.
Constituting about 0.03% of air by volume, it is produced when carbon-containing materials burn completely, and it is a product of fermentation and animal respiration. Plants use CO2 in photosynthesis to make carbohydrates. CO2 in Earth's atmosphere keeps some of the Sun's energy from radiating back into space (see greenhouse effect). In water, CO2 forms a solution of a weak acid, carbonic acid (H2CO3). The reaction of CO2 and ammonia is the first step in synthesizing urea. An important industrial material, CO2 is recovered from sources including flue gases, limekilns, and the process that prepares hydrogen for synthesis of ammonia. It is used as a refrigerant, a chemical intermediate, and an inert atmosphere; in fire extinguishers, foam rubber and plastics, carbonated beverages (see carbonation), and aerosol sprays; in water treatment, welding, and cloud seeding; and for promoting plant growth in greenhouses. Under pressure it becomes a liquid, the form most often used in industry. If the liquid is allowed to expand, it cools and partially freezes to the solid form, dry ice.
Carbonate
Any member of two classes of chemical compounds—one inorganic and the other organic—that are derived from carbon dioxide (CO2) or its water solution, carbonic acid (H2CO3).
Inorganic carbonates (MCO3 or M2CO3, where M is a metal atom of, e.g., calcium or sodium) are salts of carbonic acid. The shells and other hard parts of shellfish are calcium carbonate, as is the limestone they turn into. Many other minerals, including calcite, dolomite, and aragonite, consist of or contain carbonates. Sodium carbonate is one of the world's most important basic chemical commodities. Organic carbonates are esters of carbonic acid and various alcohol groups (methyl, ethyl, or phenyl). These are liquids used as solvents and to synthesize plastics and other compounds.
