Carbon

History

(Latin. carbo: charcoal) Carbon 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.

Properties

Carbon is found free in nature in three allotropic forms: graphite, diamond, and fullerines. A fourth form, known as "white" carbon, is now thought to exist. Ceraphite is one of the softest known materials while diamond is one of the hardest.

Graphite exists in two forms: alpha and beta. These have identical physical properties, except for their crystal structure. Naturally occurring graphites are reported to contain as much as 30% of the rhombohedral (beta) form, whereas synthetic materials contain only the alpha form. The hexagonal alpha type can be converted to the beta by mechanical treatment, and the beta form reverts to the alpha on heating it above 1000oC.

In 1969 a new allotropic form of carbon was produced during the sublimation of pyrolytic graphite at low pressures. Under free-vaporization conditions above ~2550oK, "white" carbon forms as small transparent crystals on the edges of the planes of graphite. The interplanar spacings of "white" carbon are identical to those of carbon form noted in the graphite gneiss from the Ries (meteroritic) Crater of Germany. "White" carbon is a transparent birefringent material. Little information is presently available about this allotrope.

Carbon 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 energy of the sun and stars can be attributed at least in part to the well-known carbon-nitrogen cycle.

Sources 

Carbon was not created in the big bang due to the fact that it needs a triple collision of alpha particles 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.

Carbon is very widely distributed in nature. It is found in abundance in the sun, stars, comets, and atmospheres of most planets. Carbon in the form of microscopic diamonds is found in some meteorites.

Natural diamonds are found in kimberlite of ancient volcanic "pipes," found in South Africa, Arkansas, and elsewhere. Diamonds are now also being recovered from the ocean floor off the Cape of Good Hope. About 30% of all industrial diamonds used in the U.S. are now made synthetically.

Uses

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 synthetic substances, many of which are collectively called plastics.

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. It 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. Charcoal pills are used in medicine in pill or powder form to adsorb toxins or poisons from the digestive system.

In combination, carbon is found as carbon dioxide in the atmosphere of the earth and dissolved in all natural waters. It is a component of great rock masses in the form of carbonates of calcium (limestone), magnesium, and iron. Coal, petroleum, and natural gas are chiefly hydrocarbons.

Carbon is unique among the elements in the vast number and variety of compounds it can form. With hydrogen, oxygen, nitrogen, and other elements, it forms a very large number of compounds, carbon atom often being linked to another carbon atom. There are close to ten million known carbon compounds, many thousands of which are vital to organic and life processes.

Without carbon, the basis for life would be impossible. While it has been thought that silicon might take the place of carbon in forming a host of similar compounds, it is now not possible to form stable compounds with very long chains of silicon atoms. The atmosphere of Mars contains 96.2% CO2. Some of the most important compounds of carbon are carbon dioxide (CO2), carbon monoxide (CO), carbon disulfide (CS2), chloroform (CHCl3), carbon tetrachloride (CCl4), methane (CH4), ethylene (C2H4), acetylene (C2H2), benzene (C6H6), acetic acid (CH3COOH), and their derivatives.

Isotopes

Carbon has seven isotopes. In 1961 the International Union of Pure and Applied Chemistry adopted the isotope carbon-12 as the basis for atomic weights. Carbon-14, an isotope with a half-life of 5715 years, has been widely used to date such materials as wood, archaeological specimens, etc.

Hazards

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.