Carbon

Carbon (C) is a chemical element. It is the fourth most abundant element in the universe, and it is literally the building block of life.

Carbon has an atomic number Z = 6, i.e., the carbon nucleus has charge 6e, where e is the elementary charge, and carbon takes the sixth position in the periodic table of elements. The chemical symbol of carbon is C and its electron configuration is [He]2s22p2.

One twelfth of the mass of the most abundant isotope 12C is by definition the unified atomic mass unit: A(12C) &equiv; 12 u. The atomic mass averaged over the naturally occurring isotopes is 12.0107 u.

Carbon has a melting point of 3550 °C (diamond) and a sublimation point of 3800 °C.

One of the most widely distributed and abundant of all elements, carbon is a constituent of all organic matter. Diamond, graphite, and fullerenes are non-organic compounds consisting of carbon only; these are known as allotropes of carbon. Most organic carbon compounds contain hydrogen; those that contain oxygen as well include the extensive class of carbohydrates.

Carbon is abundant in the sun, stars, comets, and atmospheres of most planets. Carbon is also found in the form of microscopic diamonds in some meteorites.

Plastics, drugs, animal and plant tissue, foods, textile, wood, paper, and many other common substances contain carbon. Fossil fuels (coal, petroleum, natural gas, etc.) are compounds of carbon.

The greenhouse gas carbon dioxide CO2 is an oxide of carbon, as is the poisonous gas carbon monoxide (CO).

Isotopes
Carbon has many isotopes (from A = 8 to 22). Two are stable, and one is very long-lived:


 * 12C Comprises about 98.89% of all carbon on Earth.
 * 13C is another natural, stable isotope of carbon comprising about 1.1% of all natural carbon on Earth.
 * 14C, an unstable isotope with a half-life of 5,715 years, is commonly used to date such materials as wood, archaeological specimens, etc.

Forms
Carbon occurs naturally in three allotropic forms: graphite, diamond, and fullerenes.

Graphite occurs in two forms: alpha and beta. They have identical physical properties and differ only in their crystalline structure. Naturally occurring graphites are reported to contain as much as 30% of the beta form, whereas synthetic materials contain only the alpha form. The alpha type can be converted to the beta by mechanical manipulation, and the beta form will revert to the alpha when it is heated above 1000 °C.

An allotropic form of carbon was produced in 1969. Ceraphite, white carbon, was produced when pyrolytic graphite was sublimated at low pressures. Under free-vaporization conditions above approximately 2550 K, "white" carbon will form as small transparent crystals. Little information is presently available about this allotrope.

Fullerene (after Richard Buckminster Fuller, also known as "bucky ball", a "soccer ball" of sixty carbon atoms) was discovered in 1985, and graphene, consisting of two-dimensional sheets of carbon, was first prepared in 2004.

Carbon cycle
Carbon is the essential element of all organic substances ranging from fossil fuels to cell components and DNA. Here on Earth, carbon circulates in various chemical components in a vast biogeochemical cycle through the lithosphere (land and Earth’s interior), hydrosphere (ocean and other bodies of water), and atmosphere.

There are two categories to this global carbon cycle: the geological cycle operating over millions of years, and the biological/physical cycle, which is much shorter and spans a few days to thousands of years.

The oceans of the Earth are dominant environments in the carbon cycle. The oceans collectively hold about 50 times as much carbon as the atmosphere. The carbon in the oceans is exchanged with the atmosphere over a period of hundreds of years. Nearly ½ of all oxygen we breathe is generated by photosynthesis of marine plants. Approximately 48% of all carbon produced by burning fossil fuel is sequestered (absorbed) into the oceans. This sequestered carbon accumulates in the deep ocean and ocean sediment.