Silicon, a lustrous, grayish-black chemical element that has both metallic and nonmetallic properties. Silicon is hard and brittle. It is a semiconductor; that is, its electrical conductivity is intermediate between that of a conductor and that of an insulator. Chemically, silicon is relatively inert at ordinary temperatures. It resists attack by all acids except hydrofluoric acid, and is not easily oxidized by air. At high temperatures, silicon can combine with many other elements.

Silicon is the second most abundant element on earth (after oxygen). It makes up about 28 per cent of the earth's crust. In nature, silicon is always found combined with other elements. It usually occurs as silica (silicon dioxide) or as silicates, compounds containing silicon, oxygen, and one or more metals. Most common rocks, soils, and clays consist mainly of silicates. Silica occurs chiefly as quartz, a common igneous rock-forming mineral and the chief constituent of sand and sandstone.

Silica is found in many plants, and is necessary to build strong cell walls. The shells of diatoms and the skeletons of certain sponges consist mainly of silica. Trace amounts of silica occur in such animal parts as feathers and hair.

Silicon (from the Latin silex, flint) was first isolated in 1823 by the Swedish chemist Jns J. Berzelius. Most commercial silicon is produced by heating sand with coke in an electric arc furnace. The silicon thus obtained, about 98 per cent pure, is used primarily for alloying purposes. Silicon of higher purity is usually prepared by heating silicon tetrachloride or trichlorosilane with hydrogen gas.


Silicon is widely used in alloys. Copper, when alloyed with silicon, becomes stronger and easier to weld; aluminum, easier to cast; and alloy steels, harder and stronger. Ferrosilicon alloys are used as deoxidizers in steelmaking and as reducing agents in preparing such metals as magnesium and chromium. Highly pure silicon crystals are used as semiconductors in such devices as transistors, power rectifiers, and solar batteries.

Silicon minerals and compounds have many commercial uses. Quartz is used as a flux in metallurgy, and in the manufacture of glass, enamels, mortar, and many other substances. Many silicates are important ore minerals. Some are used to make cement, brick, pottery, porcelain, electrical insulation, and heat-resistant fabrics. Certain silicates (such as emerald and topaz) and some forms of silica (such as opal and amethyst) are highly prized gems. Diatomite, a soft rock composed of fossilized diatom shells, is used as a filter for liquids, as a mild abrasive, and as heat insulation.

Silica gel, a porous form of silicon dioxide, is used as a drying agent. A typical use is for keeping packaged instruments dry by taking up any moisture in the package. Silicon carbide, an extremely hard substance, is used as an abrasive for grinding, cutting, and polishing metal. Sodium silicate, also called water glass, is used as a coating to preserve eggs and as an industrial adhesive. The silicones are organic silicon compounds that are important oils, resins, and rubber-forming substances.

Symbol: Si. Atomic number: 14. Atomic weight: 28.0855. Specific gravity: 2.33. Melting point: 2,570 F. (1,410 C.). Boiling point: 4,271 F. (2,355 C.). Silicon has three stable isotopes: Si-28 to Si-30. It belongs to Group IV-A of the Periodic Table and may have a valence of +2, +4, or -4.