SILICON

(Si), a nonmetallic chemical element in the carbon family (Group IVa of the periodic table). Silicon makes up 27.7 percent of the Earth's crust; it is the second most abundant element in the crust, being surpassed only by oxygen. Silicon was first isolated and described as an element in 1824 by Jns Jacob Berzelius, a Swedish chemist. Silicon does not occur uncombined in nature; but it is found in practically all rocks as well as in sand, clays, and soils, combined either with oxygen as silica (SiO2, silicon dioxide) or with oxygen and other elements (e.g., aluminum, magnesium, calcium, sodium, potassium, or iron) as silicates. Its compounds also occur in all natural waters, in the atmosphere (as siliceous dust), in many plants, and in the skeletons, tissues, and body fluids of some animals. Pure silicon is a hard, dark gray solid with a metallic lustre and with a crystalline structure the same as that of the diamond form of carbon, to which silicon shows many chemical and physical similarities. A brown, powdery form of silicon has been described that also has a microcrystalline structure. The element is prepared commercially by reducing (removing the oxygen from) the oxide by its reaction with coke in electric furnaces. On a small scale, silicon can be obtained from the oxide by reduction with aluminum. Silicon, like carbon, is relatively inactive at ordinary temperatures; but when heated it reacts vigorously with the halogens (fluorine, chlorine, bromine, and iodine) to form halides and with certain metals to form silicides. It is unaffected by acids except hydrofluoric. At red heat, silicon is attacked by water vapour or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures (2,0002,600 C [3,6004,700 F]), they form silicon carbide (Carborundum, SiC), which is an important abrasive. In union with hydrogen, silicon forms a series of hydrides, the silanes. When united with hydrocarbon groups, silicon forms a series of organic silicon compounds. Silicon's atomic structure makes it an extremely important semiconductor; highly purified silicon, doped with such elements as boron, phosphorus, and arsenic, is the basic material used in computer chips, transistors, silicon diodes, and various other electronic circuits and switching devices. Silicon of lesser purity is used in metallurgy as a reducing agent and as an alloying element in steel, brass, and bronze. The most important compounds of silicon are the dioxide (silica) and the various silicates. Silica in the form of sand and clay is used to make concrete and bricks as well as refractory materials for high-temperature applications. As the mineral quartz, the compound may be softened by heating and shaped into glassware. Silicates, most of which are insoluble in water, are employed in making glass as well as in the fabrication of enamels, pottery, china, and other ceramic materials. Sodium silicates, commonly known as water glass, or silicate of soda, are used in soaps, in the treatment of wood to prevent decay, for the preservation of eggs, as a cement, and in dyeing. Silicones are synthetic organosilicon oxides composed of the elements silicon, oxygen, carbon, and hydrogen; they are used as lubricants, hydraulic fluids, waterproofing compounds, varnishes, and enamels because, as a class, they are chemically inert and unusually stable at high temperatures. Three stable isotopes of silicon are known: silicon-28, which makes up 92.21 percent of the element in nature; silicon-29, 4.70 percent; and silicon-30, 3.09 percent. Five radioactive isotopes are known. atomic number 14 atomic weight 28.086 melting point 1,410 C (2,570 F) boiling point 2,355 C (4,270 F) density 2.33 g/cm3 oxidation state -4, (+2), +4 electron config. 2-8-4 or 1s22s22p63s23p2