Description and references
C; at. wt 12.0107; at. no. 6; valence 4. Group
IVA(14). Stable isotopes: 12 (98.892%); 13 (1.108%); radioactive
isotopes: 9-11; 14-16. Abundance in earth's crust: approx 0.027%.
Cosmic abundance: 6 atoms/atom Si. Occurs in 4 allotropic forms:
(1) diamond, q.v.; (2) graphite, q.v. or black lead; (3)
amorphous carbon such as coal, lampblack; (4) fullerenes, see Buckminsterfullerene, the only
molecular form. Comprehensive reviews: P. L. Walker, Am. Sci. 50, 259-293 (June 1962);
Holliday et al. in Comprehensive
Inorganic Chemistry vol. 1, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 1173-1294; several
authors in Kirk-Othmer Encyclopedia of Chemical
Technology vol. 4 (Wiley-Interscience, New
York, 3rd ed., 1978) pp 556-709.
Derivative
14C isotope.
Properties
Continuously formed in the earth's atm by the
bombardment of nitrogen with cosmic neutrons according to the reaction 714N + 01n → 614C + 11H. The 14C is rapidly oxidized to CO2, in this form it penetrates
into animals and plants by photosynthesis and metabolism. The 14C content of living matter is estimated at 15.3 disintegrations
per minute and per gram of carbon, corresponding to the equilibrium
reached between formation of 14C and its exchange with 12C. This equilibrium stops when the plant or animal dies,
and the 14C content begins to decrease, because the 14C decays with a half-life of 5760 years. This fact can be
used to date organic matter (not more than 40,000 years old) by comparison
with the standard 15.3 disintegrations per min per gram: M. Haissinsky,
J. P. Adloff, Radiochemical Survey of the
Elements (Elsevier, New York, 1965) pp 30-32.