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Element Fact
Iridium is the most corrosion-resistant metal known. Together with platinum, it is used to make the international prototype of the kilogram, which is the standard definition for the unit of mass.

Carbon, C

carbon sample

Atomic number: 6
Atomic mass: 12.0107(8)
Natural abundance: 180 ppm
Isotopes:
12C (12.000000000, by definition) - 98.9%
13C (13.003354838) - 1.1%
14C (14.003241988) - wide variation.

History and Production

From Latin carbo, meaning charcoal. It is an element of prehistory discovery as charcoal, soot, diamond and graphite. However, diamond and graphite are known to be different forms of element (allotrope) at the end of eighteenth century: C.W. Scheele found that graphite is a carbon in 1779 while S. Tennant found diamond is a different form of carbon in 1796. Of more recent interest is the discovery of buckyballs or fullerenes, which have a number of unusual properties. Graphite, which can be extracted in nature, or can be manufactured by heating coke with silica at about 2500C for 25-35 hours. It is used for electrodes, crucibles, motor brush, pencils etc. Very pure graphite is used as a neutron moderator in nuclear reactors. Diamond, which can be found from kimberlite pipes or produce synthetically. It has its use in jewellery, cutting, drilling and polishing. Coke, a poorly graphitized carbon is, produced from high-temperature carbonization of coal. It is used mainly in blast furnaces for steel manufacture. The 14C isotope is widely used to date materials such as woods and archeological specimens.

Physical Data

It is found in wide abundance in the universe. It is found as free elements in nature, mostly in three allotropic forms: graphite, diamond and amorphous. Graphite, black in color, is one of the softest material known, while diamond (tetrahedral) is brilliant sparkle due to its transparency and high refractive index, is the hardest known material. Occasionally, diamonds may contain inclusions, such as the blue diamond which contains aluminium. Carbon also exists in combined form, mainly as carbonates of calcium and magnesium. It also occurs as the carbon dioxide in the atmosphere. The buckyballs, consists of 60-70 carbon molecules, can withstand great pressure and trap foreign atoms inside their network of carbon.

In 1961, the International Union of Pure and Applied Chemistry (IUPAC) adopted the isotope 12C as the basis for atomic weight, which has a mass of exactly 12.000 atomic mass unit (amu). The number so chosen is arbitrary but it turns out that the lightest element, hydrogen has a sensible starting mass of about 1 amu.

Interatomic distance: 154.45 pm (diamond)
Melting point: 3547C (diamond), 3527C (graphite)
Boiling point: 4027C
Thermal conductivity/Wm-1K-1: 140 (27C)
Density/kgm-3: 3513 (diamond), 2260 (graphite), 1650 (C60 at 20C)

Standard Thermodynamic Data (graphite, gas)

Enthalpy of formation: 716.7 kJ/mol
Gibbs free energy of formation: 671.3 kJ/mol
Entropy: 158.1 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [He] 2s2 2p2
Term symbol: 3P0
Electron affinity: 121.7761 kJ/mol   Electronegativity (Pauline): 2.55
Ionization energy (first, second, third): 1086.45, 2352.63, 4620.47 kJ/mol

Chemical Properties

Carbon is one of the main building block of the organic molecules and it can form a wide variety of coumpounds in a wide variety way. It is known to form all possible (1 to 8) coordination compounds, although 3- or 4- coordinate compounds are the most common. There are already about 10 million known carbon compounds. Diamond is unreactive at room temperature, whereas graphite is chemically more reactive, due to its more open layer atomic structure. For instance, in the presence of strong oxidizing agents, sulfuric acid reacts with graphite to form a 'hydrogen sulphate', (Cx)+HSO4-.2H2SO4, in which the anions are trapped between carbon cations formed by the hexagonal-structured sheets of carbon atoms.



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