The first element of group 14 (formerly IVA) of the periodic table. Carbon is a universal constituent of living matter and the principal deposits of carbon compounds are derived from living sources; i.e., carbonates (chalk and limestone) and fossil fuels (coal, oil, and gas). It also occurs in the mineral dolomite. The element forms only 0.032% by mass of the Earth’s crust. Minute quantities of elemental carbon also occur as the allotropes graphite and diamond. A third allotrope, buckminsterfullerene (C₆₀), also exists. The industrial demand for graphite is such that it is manufactured in large quantities using the Acheson process in which coke and small amounts of asphalt or clay are raised to high temperatures. Large quantities of impure carbon are also consumed in the reductive extraction of metals. Apart from the demand for diamond as a gemstone there is a large industrial demand for low-grade small diamonds for drilling and grinding machinery. High temperature studies show that graphite and diamond can be inter-converted at 3000°C and extremely high pressures but the commercial exploitation would not be
viable. Carbon burns in oxygen to form carbon dioxide and carbon monoxide, CO. Carbon dioxide is soluble in water forming the weakly acidic carbonic acid, the parent acid of the metal carbonates. In contrast CO is barely soluble in water but it will react with alkali to give the methanoate (formate) ion:
CO + OH⁻ → HCO²⁻
Carbon will react readily with sulfur at red heat to form CS₂ but it does not react directly with nitrogen. Cyanogen, (CN)₂, must be prepared by heating covalent metal cyanides such as CuCN. Carbon will also react directly with many metals at elevated temperatures to give carbides. Carbides can also be obtained by heating the metal oxide with carbon or heating the metal with a hydrocarbon. There is a bewilderingly wide range of metal carbides, both salt-like with electropositive elements (CaC₂) and covalent with the metalloids (SiC), and there are also many interstitial carbides formed with metals such as Cr, Mn, Fe, Co, and Ni.
The compounds with C–N bonds form a significant branch of inorganic chemistry of carbon, these are hydrogen cyanide (HCN) and the cyanides, cyanic acid (HNCO) and the cyanates, and thiocyanic acid (HNCS) and the thiocyanates. Naturally occurring carbon has the isotopic composition ¹²C (98.89%), ¹³C (1.11%) and ¹⁴C (minute traces in the upper atmosphere produced by slow neutron capture by ¹⁴N atoms). ¹⁴C is used for radiocarbon dating because of its long half-life of 5730 years.
Symbol: C; m.p. 3550°C; b.p. 4830°C (sublimes); C₆₀ sublimes at 530°C; r.d. 3.51 (diamond), 2.26 (graphite), 1.65 (C₆₀) (all at 20°C); p.n. 6; r.a.m. 12.011.