BOILING POINT: 2,355°C
DENSITY: 2.329 g/cm 3
MOST COMMON IONS: Si 4+ , H 3 SiO 4 − , H 2 SiO 4 2− , HSiO 4 3− , SiO 4 4−
Silicon is the second most abundant element in Earth's crust and mantle, after oxygen. It is the seventh most abundant element in the universe. It was first obtained in elemental form by Jöns Jakob Berzelius in 1823, from reduction of a complex fluoride, K 2 SiF 6 , by potassium. It has a strong chemical affinity for electronegative elements such as oxygen and fluorine. It is always found in nature in bound form, as the oxide, SiO 2 , or in silicate minerals such as olivine, (Mg, Fe) 2 SiO 4 . The principal hydride is silane (SiH 4 ), a pyrophoric gas, and the halides (e.g., SiF 4 , SiCl 4 ) are gases or liquids. Silicate minerals form in a wide variety of crystallographic structures, i.e., a mineral's internal (repeating) structure; many of these minerals are important ceramics (e.g., heated mica produces clays used in pottery; calcium silicates are the primary components of cement). Amorphous (noncrystalline) silicates form glasses used in windows and containers. Silicon nitride, Si 3 N 4 , is an important ceramic used in turbine engines. Silicon carbide, SiC, is an ultrahard solid used in abrasives.
Silicon is extracted from quartz (SiO 2 ) sand by high temperature reduction with carbon. The crystalline element is a shiny gray semiconducting solid with the tetrahedral diamond structure. Usually doped with boron, arsenic , or phosphorus, it conducts electricity via the diffusion of electrons or positive "holes" (electron vacancies). Polymorphs of silicon formed under high pressure conditions are metallic and contain octahedral silicon; the liquid is metallic also. Amorphous silicon formed by vapor phase decomposition of silane is used to transform light into electricity in solar cells. Silicones (organic silicon compounds containing Si-O-Si linkages) are prepared from silane, silicon halides, or metal -Si alloys by hydrolysis of the silicone material to give oils, greases, synthetic rubbers, and adhesives.
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