Encyclopedia > Carbon

Carbon is a chemical element in the periodic table that has the symbol C and atomic number 6. An abundant nonmetallic, tetravalent element, carbon has three (or four) allotropic forms:

diamonds (hardest known mineral)
graphite (one of the softest substances)
fullerite (fullerenes (which has interesting structural and electrochemical properties that suit it for use in nanotechnology)
lamp black (not really an allotrope but a powdered form of graphite)

Carbon occurs in all organic life and is the basis of organic chemistry. This nonmetal also has the interesting chemical property of being able to bond with itself and a wide variety of other elements (making more than 10 million compounds). When united with oxygen it forms carbon dioxide which is absolutely vital to plant growth. When united with hydrogen, it forms various compounds called hydrocarbons which are essential to industry in the form of fossil fuels. When combined with both oxygen and hydrogen it can form many groups of compounds including fatty acids, which are essential to life, and esters, which give flavour to many fruits. The isotope carbon-14 is commonly used in radioactive dating.

Table of contents

1 Notable Characteristics

3 History

Notable Characteristics

Carbon is a remarkable element for many reasons. Its different forms include one of the softest (graphite) and one of the hardest (diamond) substances known to man. Moreover, it has a great affinity for bonding with other small atoms, including other carbon atoms, and its small size makes it capable of forming multiple bonds. These properties yield nearly ten million carbon compounds. Carbon compounds form the basis of all life on Earth and the carbon-nitrogen cycle provides some of the energy produced by the sun and other stars.

Carbon was not created in the big bang due to the fact that it needs a triple collision of alpha particles (helium nuclei) to be produced. The universe initially expanded and cooled too fast for that to be possible. It is produced, however, in the interior of stars in the horizontal branch, where stars transform a helium core into carbon by means of the triple-alpha process.

Applications

Carbon is a vital component of all known living systems, and without it life as we know it could not exist (see carbon chauvinism). The major economic use of carbon is in the form of hydrocarbons, most notably the fossil fuels methane gas and crude oil. Crude oil is used by the petrochemical industry[?] to produce, amongst others, petroleum, gasoline and kerosene, through a distillation process, in so-called refineries[?]. Crude oil forms the raw material for many synthetical substances, many of which are collectively called plastics.

Other uses:

The isotope ¹⁴C discovered February 27th 1940 is used in radiocarbon dating
Graphite is used as the "lead" in pencils, in combination with clays.
Diamonds are used for decorative purposes, and also as drill bits and other applications making use of its hardness.
Carbon is added to iron to make steel.
Carbon is used for control rods[?] in nuclear reactors.
Graphite carbon in a powdered, caked form is used as charcoal for cooking, artwork and other uses.

The chemical and structural properties of fullerenes, in the form of carbon nanotubes, has promising potential uses in the nascent field of nanotechnology.

History

Carbon (Latin carbo meaning "charcoal") was discovered in prehistory and was known to the ancients, who manufactured it by burning organic material in insufficient oxygen (making charcoal). Diamonds have long been considered rare and beautiful. The last-known allotrope of carbon, fullerenes, were discovered as byproducts of molecular beam experiments in the 1980's.

Allotropes

Four allotropes of carbon are known to exist: amorphous, graphite, diamond and fullerenes.

In its amorphous form, carbon is essentially graphite but not held in a crystalline macrostructure. It is, rather, present as a powder which is the main constituent of substances such as charcoal and lamp black[?] (soot[?]).

At normal pressures carbon takes the form of graphite, in which each atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons[?]. The two known forms of graphite, alpha (hexagonal) and beta (rhombohedral[?]), both have identical physical properties, except for their crystal structure. Graphites that naturally occur have been found to contain up to 30% of the beta form, when synthetically-produced graphite only contains the alpha form. The alpha form can be converted to the beta form through mechanical treatment and the beta form reverts back to the alpha form when it is heated above 1000 °C.

Because of the delocalization of the pi-cloud[?], graphite conducts electricity. The material is soft and the sheets, frequently separated by other atoms, are held together only by van der Waals forces, so easily slip past one another.

At very high pressures carbon has an allotrope called diamond, in which each atom is bonded to four others. Diamond has the same cubic structure as silicon and germanium and, thanks to the strength of the carbon-carbon bonds, is together with the isoelectronic boron nitride (BN) the hardest substance in terms of resistance to scratching. The transition to graphite at room temperature is so slow as to be unnoticeable. Under some conditions, carbon crystallizes as Lonsdaleite, a form similar to diamond but hexagonal.

Fullerenes have a graphite-like structure, but instead of purely hexagonal packing, also contain pentagons (or possibly heptagons) of carbon atoms, which bend the sheet into spheres, ellipses or cylinders. The properties of fullerenes (also called "buckyballs" and "buckytubes") have not yet been fully analyzed. All the names of fullerenes are after Buckminster Fuller, developer of the geodesic dome, which mimics the structure of "buckyballs".

Occurrence

There are nearly ten million carbon compounds that are known to science and many thousands of these are vital to life processes and very economically important organic-based reactions. This element is abundant in the sun, stars, comets, and in the atmospheres of most planets. Some meteorites contain microscopic diamonds that were formed when the solar system was still a protoplanetary disk. In combination with other elements, carbon is found the earth's atmosphere and dissolved in all bodies of water. With smaller amounts of calcium, magnesium, and iron, it is a major component of very large masses carbonate rock (limestone, dolomite, marble etc.). When combined with hydrogen, carbon form coal, petroleum, and natural gas which are called hydrocarbons.

Graphite is found in large quantities in New York and Texas, the United States; Russia; Mexico; Greenland and India.

Natural diamonds occur in the mineral kimberlite found in ancient volcanic "necks," or "pipes". Most diamond deposits are in Africa, notably in South Africa, Namibia, Botswana, the Republic of the Congo and Sierra Leone. There are also deposits in Canada, the Russian Arctic, Brazil and in Northern and Western Australia.

Compounds

(this article only deals with inorganic compounds of carbon; see also organic chemistry)

The most prominent oxide of carbon is carbon dioxide, CO₂. This is a minor component of the Earth's atmosphere, produced and used by living things, and a common volatile elsewhere. In water it forms trace amounts of carbonic acid, H₂CO₃, but as most compounds with multiple single-bonded oxygens on a single carbon it is unstable. Through this intermediate, though, resonance-stabilized carbonate ions are produced. Some important minerals are carbonates, notably calcite. Carbon disulfide, CS₂, is similar.

The other oxides are carbon monoxide, CO, and the uncommon carbon suboxide, C₃O₂. Carbon monoxide is formed by incomplete combustion, and is a colorless, odorless gas. The molecules each contain a triple bond and are fairly polar, resulting in a tendency to bind permanently to hemoglobin molecules, so that the gas is highly poisonous. Cyanide, CN-, has a similar structure and behaves a lot like a halide ion; the nitride cyanogen[?], (CN)₂, is related.

With strong metals carbon forms either carbides, C^-, or acetylides, C₂^2-; these are associated with methane and acetylene, both incredibly pathetic acids. All in all, with an electronegativity of 2.5, carbon prefers to form covalent bonds[?]. A few carbides are covalent lattices, like carborundum, SiC, which resembles diamond.

Isotopes

In 1961 the International Union of Pure and Applied Chemistry adopted the isotope carbon-12 for basis for atomic weights. Carbon-14 is a radioisotope with a half-life of 5715 years and has been used extensively for radiocarbon dating wood, archaeological sites and specimens.

Carbon has two stable, naturally-occurring isotopes: C-12 (98.89%) and C-13 (1.11%). Ratios of these isotopes are reported in ? relative to the standard VPDB (Vienna Pee Dee Belemnite from the Peedee Formation of South Carolina). The dC-13 of the atmosphere is -7?. During photosynthesis, the carbon that becomes fixed[?] in plant tissue is significantly depleted in C-13 relative to the atmosphere.

There is two mode distribution in the dC-13 values of terrestrial plants resulting from differences in the photosynthetic reaction used by the plant. Most terrestrial plants are C3 pathway plants[?] and have dC-13 values range from -24 to -34?. A second category of plants (C4 pathway plants[?]), composed of aquatic plants, desert plants, salt marsh plants, and tropical grasses, have dC-13 values that range from -6 to -19. An intermediate group (CAM plants[?]) composed of algae and lichens has dC-13 values range from -12 to -23?. The dC-13 of plants and organisms can provide useful information about sources of nutrients and food web relations.

Precautions

Compounds of carbon have a wide range of toxic action. Carbon monoxide (CO), which is present in the exhaust of combustion engines, and cyanide (CN^-), which is sometimes in mining pollution, are extremely toxic to mammals. Many other carbon compounds are not toxic and are in fact absolutely essential for life. Organic gases such as ethene (CH₂=CH₂), ethyne (HCCH), and methane (CH₄) are dangerously explosive and flammable when mixed with air.

External Links

WebElements.com - Carbon (http://www.webelements.com/webelements/elements/text/C/index)
EnvironmentalChemistry.com - Carbon (http://environmentalchemistry.com/yogi/periodic/C)
It's Elemental - Carbon (http://education.jlab.org/itselemental/ele006)

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