Title: Silicon
CAS Registry Number: 7440-21-3
Literature References: Si; at. wt 28.0855; at. no. 14; valence 4, also 2. Group IVA (14). Three naturally occurring isotopes: 28 (92.18%); 29 (4.71%); 30 (3.12%); artificial isotopes: 25-27; 31; 32. Does not occur free in nature; found as silica (quartz, sand, sandstone) or as silicate (feldspar or orthoclase, kaolinite, anorthite, etc.). Constitutes about 27.6% of the earth's crust; second most abundant element on earth, oxygen being first. Prepd industrially by carbon reduction of silica in an electric arc furnace. Purification by zone refining (see ref. under Germanium). Very pure silicon is obtained by decompn of silicon tetraiodide: Litton, Anderson, J. Electrochem. Soc. 101, 287 (1954); Chem. Eng. News 34, 5007 (1956). From silicon tetrachloride: Lyon et al., Trans. Electrochem. Soc. 96, 359 (1949); Klyuchnikov, J. Appl. Chem. USSR 29, 139 (1956). By thermal decompn of a chlorosilane: Schering, US 3041144 (1962 to Siemens-Schuckertwerke). Review of silicon and its compounds: Rochow, "Silicon" in Comprehensive Inorganic Chemistry vol. 1, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 1323-1467; W. Runyan in Kirk-Othmer Encyclopedia of Chemical Technology vol. 20 (Wiley-Interscience, New York, 3rd ed., 1982) pp 826-845. The uses of silicon compounds in organic chemistry: E. W. Colvin, Chem. Soc. Rev. 7, 15 (1978); I. Fleming, ibid. 10, 83 (1981); L. A. Paquette, Science 217, 793 (1982).
Properties: Black to gray, lustrous, needle-like crystals or octahedral platelets (cubic system). The amorphous form is a dark brown powder. Poor conductor of electricity. d425 2.33. mp 1410°. Average heat capacity (16-100°): 0.1774 cal/g/°C. Lattice constant (25°): 5.41987 ´ 10-8 cm. Compressibility (V/V0) at 25 ´ 103 kg/cm2: 0.978; at 100 ´ 103 kg/cm2: 0.940, Gmelins, Silicon (8th ed.) 15B (1959) p 57. Dielectric const: 13. Covalent bond ionization energy at 0 K = 1.2 ev. Band gap: 1.106 ev. Impurity atom ionization energy: ~0.04 ev. Intrinsic resistivity at 300 K = 0.23 megohm. Electron mobility at 300 K: 1500 cm2/volt/sec. Hole mobility at 300 K: 500 cm2/volt/sec. Intrinsic charge density at 300 K: 1.5 ´ 1010. Electron diffusion constant at 300 K: 38. Hole diffusion constant at 300 K: 13. Practically insol in water. Attacked by hydrofluoric or a mixture of hydrofluoric and nitric acids (depending upon cryst modifications). Soluble in molten alkali oxides. Burns in fluorine, chlorine.
Melting point: mp 1410°
Density: d425 2.33
CAUTION: Potential symptoms of overexposure are irritation of eyes, skin, upper respiratory system; cough. See NIOSH Pocket Guide to Chemical Hazards (DHHS/NIOSH 97-140, 1997) p 278.
Use: In making silanes and silicones, the Si-C bond being about as strong as a C-C bond. In the manuf of transistors, silicon diodes and similar semiconductors. For making alloys such as ferrosilicon, silicon bronze, silicon copper. As a reducing agent like aluminum in high temp reactions.

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