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Silicon
Silicon is a chemical Aluminum - Silicon - Phosphorus element in the periodic table that has the C symbol Si and atomic Si number 14. A tetravalent GeÊÊ [Image:Si-TableImage.png] metalloid, silicon is Ê Full table less reactive than its Ê chemical analog carbon. General It is the second most abundant element in the Name, Symbol, Number Silicon, Si, 14 Earth's crust, making up Series metalloid 25.7% of it by weight. Group, Period, Block 14 (IVA), 3, p It occurs in clay, feldspar, granite, Density, Hardness 2330 kg/m3, 6.5 quartz and sand, mainly Appearance dark grey, bluish tinge in the form of silicon dioxide (also known as Atomic Properties silica) and silicates Atomic weight 28.0855 amu (compounds containing Atomic radius (calc.) 110 (111)pm silicon, oxygen and metals). Silicon is the Covalent radius 111 pm principal component of van der Waals radius 210 pm glass, semiconductors, cement, ceramics and Electron configuration [Ne]3s2 3p2 silicones, the latter a e- 's per energy level 2, 8, 4 plastic substance often confused with silicon. Oxidation states (Oxide) 4 (amphoteric) Crystal structure Cubic face centered Physical Properties State of matter solid (nonmagnetic) Melting point 1687 K (2577 ¡F) Boiling point 3173 K (5252 ¡F) Molar volume 12.06 ×10-3 m3/mol Heat of vaporization 384.22 kJ/mol Heat of fusion 50.55 kJ/mol Vapor pressure 4.77 Pa at 1683 K Speed of sound __ m/s at __ K Miscellaneous Electronegativity 1.90 (Pauling scale) Notable Characteristics Specific heat capacity 700 J/(kg*K) In its crystalline form, Electrical conductivity 2.52 10-4/m ohm silicon has a metallic Thermal conductivity 148 W/(m*K) luster and a grayish color. Even though it is 1st ionization potential 786.5 kJ/mol a relatively inert 2nd ionization potential 1577.1 kJ/mol element, silicon still reacts with halogens and 3rd ionization potential 3231.6 kJ/mol dilute alkalis, but most acids, (except for 4th ionization potential 4355.5 kJ/mol hydrofluoric acid) do 5th ionization potential 16091 kJ/mol not affect it. Elemental silicon transmits more 6th ionization potential 19805 kJ/mol than 95% of all 7th ionization potential 23780 kJ/mol wavelengths of infrared light. 8th ionization potential 29287 kJ/mol Applications 9th ionization potential 33878 kJ/mol 10th ionization potential 38726 kJ/mol Silicon is a very useful element that is vital to Most Stable Isotopes many human industries. Silicon dioxide in the iso NA half-life DM DE MeV DP form of sand and clay is 28Si 92.23% Si is stable with 14 neutrons an important ingredient of concrete and brick 29Si 4.67% Si is stable with 15 neutrons and is also used to 30Si 3.1% Si is stable with 16 neutrons produce Portland cement. Silicon is a very 32Si {syn} 276 y Beta- 0.224 32P important element for plant and animal life. SI units & STP are used except where noted. Diatoms extract silica from water to build their protective cell walls. Other uses: * It is a refractory material used in high-temperature material production and its silicates are used in making enamels and pottery. * Silicon is an important constituent of some steels. * Silica from sand is a principal component of glass. Glass can be made into a great variety of shapes and is used to make window glass, containers, and insulators, among many other uses. * Silicon carbide is one of the most important abrasives. * Ultrapure silicon can be doped with arsenic, boron, gallium, or phosphorus to make silicon more conductive for use in transistors, solar cells and other semiconductor devices which are used in electronics and other high-tech applications. * Silicon can be used in lasers to produce coherent light with a wavelength of 4560 angstroms. * Silicones are flexible compounds containing silicon-oxygen and silicon-carbon bonds; they are widely used in applications such as artificial breast implants and contact lenses. * Hydrogenated amorphous silicon has shown promise in the production of low-cost, large-area electronics and solar cells. * Silica is a major ingredient in bricks because of its low chemical activity. History Silicon (Latin silex, silicis meaning flint) was first identified by Antoine Lavoisier in 1787, and was later mistaken by Humphry Davy in 1800 for a compound. In 1811 Gay Lussac and Thenard probably prepared impure amorphous silicon through the heating of potassium with silicon tetrafluoride. In 1824 Berzelius prepared amorphous silicon using approximately the same method of Lussac. Berzelius also purified the product by repeatedly washing it. Occurrence Silicon is a principal component of aerolites which are a class of meteoroids and also of tektites which is a natural form of glass. Measured by weight, silicon makes up 25.7% of the earth's crust and after oxygen is also the second most abundant element. Elemental silicon is not found in nature. It occurrs most often as oxides and as silicates. Sand, amethyst, agate, quartz, rock crystal, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, asbestos, feldspar, clay, hornblende, and mica are a few of the many silicate minerals. Production Silicon is commercially prepared by the heating of high-purity silica in an electric arc furnace using carbon electrodes. At temperatures over 1900¡C, the carbon reduces the silica to silicon according to the chemical equation SiO2 + C → Si + CO2 Liquid silicon collects in the bottom of the furnace, and is then drained and cooled. The silicon produced via this process is called metallurgical grade silicon and is at least 99% pure. In 1997, metallurgical grade silicon cost about $ 0.50 per g. Purification The use of silicon in semiconductor devices demands a much greater purity than afforded by metallurgical grade silicon. Historically, a number of methods have been used to produce high-purity silicon. Physical methods Early silicon purification techniques were based on the fact that if silicon is melted and re-solidified, the last parts of the mass to solidify contain most of the impurities. The earliest method of silicon purification, first described in 1919 and used on a limited basis to make radar components during World War II, involved crushing metallurgical grade silicon and then partially dissolving the silcon powder in an acid. When crushed, the silicon cracked so that the weaker impurity-rich regions were on the outside of the resulting grains of silicon. As a result, the impurity-rich silicon was the first to be dissolved when treated with acid, leaving behind a more pure product. In zone melting, the first silicon purification method to be widely used industrially, rods of metallurgical grade silicon were heated to melt at one end. Then, the heater was slowly moved down the length of the rod, keeping a small length of the rod molten as the silicon cooled and resolidified behind it. Since most impurities tend to remain in the molten region rather than resolidify, when the process was complete, most of the impurities in the rod had been moved into end that was the last to be melted. This end was then cut off and discarded, and the process repeated if a still higher purity was desired. Chemical methods Today, silicon is instead purified by converting it to a silicon compound that can be more easily purified tht silicon itself, and then converting that silicon compound back into pure silcon. Trichlorosilane is the silicon compound most commonly used as the intermediate, although silicon tetrachloride and silane are also used. When these gases are blown over silicon at high temperature, they decompose to high-purity silicon. In the Siemens process, high-purity silicon rods are exposed to trichlorosilane at 1150¡C. The trichlorosilane gas decomposes and deposits additional silicon onto the rods, enlarging them according to chemical reactions like 2 HSiCl3 → Si + 2 HCl + SiCl4 Silicon produced from this and similar processes is called polycrystalline silicon. Polycrystalline silicon typically has impurity levels of 1 part per billion or less. At one time, DuPont produced ultrapure silicon by reacting silicon tetrachloride with high-purity zinc vapors at 950¡C, producing silicon according to the chemical equation SiCl4 + 2 Zn → Si + 2 ZnCl2 However, this technique was plauged with practical problems (such as the zinc chloride byroduct solidifying and clogging lines) and was evenutally abandoned in favor of the Siemens process. Crystallization The Czochralski process is often used to make high-purity single silicon crystals for use in solid-state/semiconductor devices. Isotopes Silicon has nine isotopes, with mass numbers from 25-33. Si-28 (the most abundant isotope, at 92.23%), Si-29 (4.67%), and Si-30 (3.1%) are stable; Si-32 is a radioactive isotope produced by argon decay. Its half-life, after much argument, has been determined to be approximately 276 years, and it decays by beta emission to P-32 (which has a 14.28 year half-life) and then to S-32. Precautions A serious lung disease known as silicosis often occurred in miners, stonecutters, and others who were engaged in work where siliceous dust was inhaled in great quantities. Miscellaneous Information Because Silicon is an important element in semiconductor and high-tech devices, the high-tech region of Silicon Valley, California, is named after this element.
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