Properties

Carbon atom

Carbon is the sixth most abundant element in the universe and is unique due to its dominant role in the chemistry of life and in the human economy. It is a nonmetallic element having the symbol C, the atomic number 6, an atomic weight of 12.01115, and a melting point about 360ºC. There are four known allotropes of carbon: amorphous, graphite, diamond, and fullerene. A new fifth allotrope of carbon was recently produced, a spongy solid called a magnetic carbon “nanofoam” that is extremely lightweight and attracted to magnets.

History

The name derives from the Latin carbo, for "charcoal". It was known in prehistoric times in the form of charcoal and soot. In 1797, the English chemist Smithson Tennant proved that diamond is pure carbon. 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.

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

Uses

Due to carbon’s unusual chemical property of being able to bond with itself and a wide variety of other elements, it forms nearly 10 million known compounds. Carbon is present as carbon dioxide in the atmosphere and dissolved in all natural waters. It is a component of rocks as carbonates of calcium (limestone), magnesium, and iron.

The fossil fuels (coal, crude oil, natural gas, oils sands, and shale oils) are chiefly hydrocarbons. Carbon is the active element of photosynthesis and the key structural component of all living matter. The isotope carobon-12 is used as the basis for atomic weights. Carbon-14, a radioactive isotope with a half-life of 5,730 years, is used to date such materials as wood and archeological specimens. In 1960, W.F. Libby was awarded the Nobel Prize in Chemistry for developing the carbon dating method.

Organic chemistry, a major subfield of chemistry, is the study of carbon and its compounds. Because carbon dioxide is a principal greenhouse gas, the global carbon cycle has become a focus of scientific inquiry in relation to global warming, and the management of carbon dioxide emissions from the combustion of fossil fuels is a central technological, economic, and political concern.

Forms

Carbon is found free in nature in three allotropic forms: amorphous, graphite, and diamond. 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 1000°C.

In 1969 a new allotropic form of carbon was produced during the sublimation of pyrolytic graphite at low pressures. Under free-vaporization conditions above ~ 2550K, "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 transparent birefringent material.

Carbon found in organic molecules—molecules that contain carbon atoms bonded to hydrogen atoms and to other carbon atoms—is called organic carbon. Carbon is the most abundant element found in organisms. For this reason, carbon is considered the fundamental building block of all life. Plants acquire carbon from the atmosphere through photosynthesis. Using inorganic carbon in the form of carbon dioxide (CO₂) from the atmosphere and energy from sunlight, plants convert CO₂ to organic carbon as they produce stems, leaves, and roots. Carbon may also be converted from inorganic to organic forms using chemical energy in the absence of light by chemoautotrophs. Heterotrophs—organisms such as animals, fungi, and many types of bacteria that cannot synthesize their own food from carbon dioxide—obtain their carbon from organic compounds.

Compounds

In combination, carbon is found as carbon dioxide (CO₂) 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 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 many isotopes, but just three are stable enough to exist in detectable amounts in nature. Carbon-12, a stable (non-radioactive) isotope, comprises nearly 99% of all carbon on Earth. In 1961 the International Union of Pure and Applied Chemistry adopted the isotope carbon-12 as the basis for atomic weights. Carbon-13, also a stable isotope, is the next most abundant, comprising slightly more than 1% of all carbon on Earth. Carbon-14 is the most abundant radioactive isotope of carbon at 1 part per trillion. It has a half life of 5730 years and has been widely used to date such materials as wood, archaeological specimens, etc, through radiocarbon dating. All other isotopes of carbon are highly unstable and extremely rare.

Biological Implications

Carbon is a critical element to all life. It is one of the six bulk elements and is the second-most common element in the human body. By mass it is the most abundant constituent of all the major molecules that organisms are formed from, including nucleic acids (e.g., DNA), proteins, carbohyrdrates, and lipids. As a result, living organisms are intimately involved in the carbon cycle. Carbon dioxide is converted through various life processes into many different carbon compounds, which are then converted to fossil fuels by decay processes, which, when burned, reforms carbon dioxide. Some carbon compounds such as carbon monoxide (CO) or the cyanide ion CN^- are very dangerous to human health.

Further Reading

• The Chemistry of Carbon, New York University.
• Understanding the Global Carbon Cycle, The Woods Hole Research Center.