silicon

/sil"i keuhn, -kon'/, n. Chem.

a nonmetallic element, having amorphous and crystalline forms, occurring in a combined state in minerals and rocks and constituting more than one fourth of the earth's crust: used in steelmaking, alloys, etc. Symbol: Si; at. wt.: 28.086; at. no.: 14; sp. gr.: 2.4 at 20°C.

[1817; SILIC(A) + -on, as in carbon and boron]

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Nonmetallic to semimetallic chemical element, chemical symbol Si, atomic number 14.

Second only to oxygen in abundance in Earth's crust, it never occurs free but is found in almost all rocks and in sand, clay, and soils, combined with oxygen as silica (silicon dioxide, SiO₂) or with oxygen and metals as silicate minerals. It occurs in many plants and some animals. Pure silicon is a hard, dark gray solid with a metallic lustre and the same crystal structure as diamond. It is an extremely important semiconductor; doped (see dopant) with boron, phosphorus, or arsenic, it is used in various electronic circuit and switching devices, including computer chips, transistors, and diodes. Silicon is also used in metallurgy as a reducing (see reduction) agent and in steel, brass, and bronze. Its usual valence in compounds is 4. Silica is used in the form of sand and clay for many purposes; as quartz, it may be heated to form special glasses. Silicates are used in making glass, enamels, and ceramics; sodium silicates (water glass) are used in soaps, wood treatment, cements, and dyeing. See also silane; silicone.

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▪ chemical element

Introduction

 a nonmetallic chemical element in the carbon family (Group 14 [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.

      The name silicon derives from the Latin silex or silicis, meaning “flint” or “hard stone.” Amorphous elemental silicon was first isolated and described as an element in 1824 by Jöns Jacob Berzelius, a Swedish chemist. Impure silicon had already been obtained in 1811. Crystalline elemental silicon was not prepared until 1854, when it was obtained as a product of electrolysis. In the form of rock crystal, however, silicon was familiar to the predynastic Egyptians, who used it for beads and small vases; to the early Chinese; and probably to many others of the ancients. The manufacture of glass containing silica was carried out both by the Egyptians—at least as early as 1500 BCE—and by the Phoenicians. Certainly many of the naturally occurring compounds called silicates were used in various kinds of mortar for construction of dwellings by the earliest people.

Occurrence and distribution

      On a weight basis, the abundance of silicon in the crust of the Earth is exceeded only by oxygen. Estimates of the cosmic abundance of other elements often are cited in terms of the number of their atoms per 10⁶ atoms of silicon. Only hydrogen, helium, oxygen, neon, nitrogen, and carbon exceed silicon in cosmic abundance. Silicon is believed to be a cosmic product of alpha-particle absorption, at a temperature of about 10⁹ K, by the nuclei of carbon-12, oxygen-16, and neon-20. The energy binding the particles that form the nucleus of silicon is about 8.4 million electron volts (MeV) per nucleon (proton or neutron). Compared with the maximum of about 8.7 million electron volts for the nucleus of iron, almost twice as massive as that of silicon, this figure indicates the relative stability of the silicon nucleus.

      Pure silicon is too reactive to be found 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. The oxidized form, as silicon dioxide and particularly as silicates, is also common in the Earth's crust and is an important component of the Earth's mantle. 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.

      In compounds, silicon dioxide occurs both in crystalline minerals (e.g., quartz, cristobalite, tridymite) and amorphous or seemingly amorphous minerals (e.g., agate, opal, chalcedony) in all land areas. The natural silicates are characterized by their abundance, wide distribution, and structural and compositional complexities. Most of the elements of the following groups in the periodic table are found in silicate minerals: Groups 1–6, 13, and 17 (I–IIIa, IIIb–VIb, and VIIa). These elements are said to be lithophilic, or stone-loving. Important silicate minerals include the clays, feldspar, olivine, pyroxene, amphiboles, micas, and zeolites.

Properties of the element

      Elemental silicon is produced commercially by the reduction of silica (SiO₂) with coke in an electric furnace, and the impure product is then refined. On a small scale, silicon can be obtained from the oxide by reduction with aluminum. Almost pure silicon is obtained by the reduction of silicon tetrachloride or trichlorosilane. For use in electronic devices, single crystals are grown by slowly withdrawing seed crystals from molten silicon.

      Pure silicon is a hard, dark gray solid with a metallic lustre and with a octahedral crystalline structure the same as that of the diamond form of carbon, to which silicon shows many chemical and physical similarities. The reduced bond energy in crystalline silicon renders the element lower melting, softer, and chemically more reactive than diamond. A brown, powdery, amorphous form of silicon has been described that also has a microcrystalline structure.

      Because silicon forms chains similar to those formed by carbon, silicon has been studied as a possible base element for silicon organisms. The limited number of silicon atoms that can catenate, however, greatly reduces the number and variety of silicon compounds compared with those of carbon. The oxidation–reduction reactions do not appear to be reversible at ordinary temperatures. Only the 0 and +4 oxidation states of silicon are stable in aqueous systems.

      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. As is true with carbon, the bonds in elemental silicon are strong enough to require large energies to activate, or promote, reaction in an acidic medium, so 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 (silica). When silicon and carbon are combined at electric furnace temperatures (2,000°–2,600° C [3,600°–4,700° F]), they form silicon carbide (carborundum, SiC), which is an important abrasive. With hydrogen, silicon forms a series of hydrides, the silanes. When combined with hydrocarbon groups, silicon forms a series of organic silicon compounds.

      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.

      Elemental silicon and most silicon-containing compounds appear to be nontoxic. Indeed, human tissue often contains 6 to 90 milligrams of silica (SiO₂) per 100 grams dry weight, and many plants and lower forms of life assimilate silica and use it in their structures. Inhalation of dusts containing alpha SiO₂, however, produces a serious lung disease called silicosis, common among miners, stonecutters, and ceramic workers, unless protective devices are used.

Uses

      Silicon's atomic structure makes it an extremely important semiconductor (see crystal: Electric properties (crystal)). Addition of an element such as boron, an atom of which can be substituted for a silicon atom in the crystal structure but which provides one less valence electron (boron is an acceptor atom) than silicon, allows silicon atoms to lose electrons to it. The positive holes created by the shift in electrons allow extrinsic semiconduction of a type referred to as positive (p). Addition of an element such as arsenic, an atom of which can also be substituted for a silicon atom in the crystal but which provides an extra valence electron (arsenic is a donor atom), releases its electron within the lattice. These electrons allow semiconduction of the negative (n) type. Highly purified silicon, doped (infused) with such elements as boron, phosphorus, and arsenic, is the basic material used in computer chips, transistors, silicon diodes, liquid crystal displays, and various other electronic and switching devices. If p-silicon and n-silicon wafers are joined, in a manner called the p–n junction, and placed in sunlight, the absorbed energy causes electrons to move across the junction and an electric current to flow in an external circuit connecting the two wafers. Such a solar cell is a source of energy for space devices.

      Silicon of lesser purity is used in metallurgy as a reducing agent and as an alloying element in steel, aluminum, 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. Silica (silicon dioxide) is useful as an abrasive, in the production of glass and other ceramic bodies, and as an adsorbent. 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. Both naturally occurring and synthetically produced silicates are important in building materials, absorbents, and ion exchangers. 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.

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.

1s²2s²2p⁶3s²3p²

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