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Carbon

From Academic Kids

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 several allotropic forms:

  • diamond (hardest known mineral). Structure: each atom is bonded tetrahedrally to four others, making a 3-dimensional network of puckered six-membered rings of atoms.
  • graphite (one of the softest substances). Structure: each atom is bonded trigonally to three other atoms, making a 2-dimensional network of flat six-membered rings; the flat sheets are loosely bonded.
  • fullerenes. Structure: comparatively large molecules formed completely of carbon bonded trigonally, forming spheroids (of which the most well-known and simplest is the buckminsterfullerene or buckyball).
  • ceraphite (an extremely soft surface). The structure is not certain.
  • lonsdaleite (a corruption of diamond). Structure: similar to diamond, but forming a hexagonal crystal lattice.
  • amorphous carbon (a glassy substance). Structure: an assortment of carbon molecules in a non-crystalline, irregular, glassy state.
  • carbon nanofoam (an extremely light magnetic web). Structure: a low-density web of graphite-like clusters, in which the atoms are bonded trigonally in six- and seven-membered rings.
  • carbon nanotubes (tiny tubes). Structure: each atom is bonded trigonally in a curved sheet that forms a hollow cylinder.

Lamp black consists of small graphitic areas. These areas are randomly distributed, so the whole structure is isotropic.

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'Glassy carbon' is isotropic and contains a high proportion of closed porosity. Unlike normal graphite, the graphitic layers are not stacked like pages in a book, but have a more random arrangement.

Carbon fibers are similar to glassy carbon. Under special treatment (stretching of organic fibers and carbonization) it is possible to arrange the carbon planes in direction of the fiber. Perpendicular to the fiber axis there is no orientation of the carbon planes. The result are fibers with a higher specific strength than steel.

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, forming nearly 10 million known 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 flavor to many fruits. The isotope carbon-14 is commonly used in radioactive dating.

Contents

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. Because of these properties, carbon is known to form nearly ten million different compounds, the large majority of all chemical 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. It was also created in a multi atomic state.

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(petroleum). Crude oil is used by the petrochemical industry to produce, amongst others, gasoline and kerosene, through a distillation process, in refineries. Crude oil forms the raw material for many synthetic substances, many of which are collectively called plastics.

Other uses

  • The isotope Carbon-14 was discovered in February 27 1940 and is used in radiocarbon dating.
  • Some smoke detectors use tiny amounts of a radioactive isotope of carbon as source of ionizing radiation (Most smoke detectors of this type use an isotope of Americium)
  • Graphite is combined with clays to form the 'lead' used in pencils.
  • Diamond is 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 as a moderator in nuclear reactors.
  • Graphite carbon in a powdered, caked form is used as charcoal for cooking, artwork and other uses.
  • Charcoal pills are used in medicine in pill or powder form to adsorb toxins or poisons from the digestive system.

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. One of the last-known allotropes of carbon, fullerenes, were discovered as byproducts of molecular beam experiments in the 1980s.

Allotropes

The allotropes of carbon are the different molecular configurations (allotropes) that pure carbon can take.

The three relatively well-known allotropes of carbon are amorphous, graphite, and diamond. Several exotic allotropes have also been synthesized or discovered, including fullerenes, carbon nanotubes, and lonsdaleite.

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, lamp black (soot) and activated carbon.

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 forms 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".

A nanofoam allotrope has been discovered which is ferromagnetic.

Carbon allotropes include:

The system of carbon allotropes spans a range of extremes.

Between diamond and graphite:

  • Diamond is hardest mineral known to man, but graphite is one of the softest.
  • Diamond is the ultimate abrasive, but graphite is a very good lubricant.
  • Diamond is an excellent electrical insulator, but graphite is a conductor of electricity.
  • Diamond is usually transparent, but graphite is opaque.
  • Diamond crystallizes in the isometric system but graphite crystallizes in the hexagonal system.

Between amorphous carbon and nanotubes:

  • Amorphous carbon is among the easiest materials to synthesize, but carbon nanotubes are extremely expensive to make.
  • Amorphous carbon is completely isotropic, but carbon nanotubes are among the most anisotropic materials ever produced.

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 forms 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.

Organic compounds

Main article: 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, C22-; these are associated with methane and acetylene, both very weak 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.

Carbon chains

Hydrocarbons are composed of a chain of carbon atoms, saturated by hydrogen atoms. Volatile oils have shorter chains. Fats have longer chain lengths, and waxes have extremely long chains.

Carbon cycle

Main article: carbon cycle

Under terrestrial conditions, conversion of one isotope to another is very rare. Therefore, for practical purposes, the amount of carbon on Earth is constant. Thus processes that use carbon must obtain it from somewhere, and dispose of it somewhere. The paths that carbon follows in the environment are called the carbon cycle. For example, plants draw carbon dioxide out of the environments and use it to build biomass. Some of this biomass is eaten by animals, where some of it is exhaled as carbon dioxide. The carbon cycle is considerably more complicated than this short loop; for example, some carbon dioxide is dissolved in the oceans; dead plant or animal matter may become sedimentary rock, and so forth.

Isotopes

Carbon has two stable, naturally-occurring isotopes: carbon-12, or 12C, (98.89%) and carbon-13, or 13C, (1.11%), and one unstable, naturally-occurring, radioisotope; carbon-14 or 14C. There are 15 known isotopes of carbon and the shortest-lived of these is 8C which decays through proton emission and alpha decay. It has a half-life of 1.98739x10-21 s.

In 1961 the International Union of Pure and Applied Chemistry adopted the isotope carbon-12 as the basis for atomic weights.

Carbon-14 has a half-life of 5700 y and has been used extensively for radiocarbon dating wood, archaeological sites and specimens.

Precautions

Carbon is relatively safe. Inhalation of fine soot in large quantities can be dangerous. Carbon may catch fire at very high temperatures and burn vigorously (as in the Windscale fire).

There are a tremendous number of carbon compounds; some are lethally poisonous (cyanide, CN-), some are essential to life (dextrose), and some are both (carbon dioxide, CO2).

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