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What is Silicon?

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Historical Information of Silicon

Silicon was discovered by Jons Jacob Berzelius, at Sweden in 1842.The origin of the name silicon came from the Latin word 'silicis' meaning 'flint'
Silicon, which is the second most abundant element by weight in the earth crust , is a well known element .It occurs very widely , as silica SiO2 (sand and quartz), and in a wide variety of silicate minerals and clays.
It is present in the sun and stars and is a principal component of a class of meteorites
known as aerolites . Silicon make up 25.7 % of the earth crust by weight, and is the second most abundant element, exceeded only by oxygen . It is found largely as silicon oxides such as sand (silica), quartz, rock crystal, amethyst, agate, flint, jasper and opal. Silicon is found also in minerals such as asbestos , clay and mica.
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Sun (relative to H=1x1012): 4.47x107
Earth crust/p.p.m.: 277 000
Alantic surface: 0.03
Alantic deep: 0.82
Pacific deep: 4.09
Residence time/years: 30 000
Classification : recycled
Oxidation state: IV
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Silicon is important in plant and animal life. Diatoms in both fresh and salt water extract silica from the water to use as a component of their cell walls. Silicon is an important ingredient in steel. Silicon carbide is one of the most important abrasives. Workers in environments where silicaceous dust is breathed may develop a serious lung disease known as silicosis.
Hydrolysis and condensation of substituted chlorosilanes can be used to produce a very great number of polymeric products, or silicones. These range from liquids to hard, glasslike solids with many useful properties. The uses of Silicon will be discussed later in Section
Silicon is a dark grey with bluish tinge substance , with its standard state as solid
At 298K . Elemental silicon transmits more than 95% of all wavelengths of infrared and has been used in lasers to produce coherent light at 465nm.

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Melting point/K 1683
Boiling point/K 2628
Latent heat of fusion/kJ mol-1 39.6
Latent heat of vaporisation/kJ mol-1 383.3
Density/kg m-3 2329[293K] ; 2525[liquid at m.p.]
Thermal conductivity/Wm-1K-1 148 [300 K]
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Electronegativities can be used to obtain a qualitative , if not a quantitative , measure of the relative charges on atoms in molecules. And because the charge distribution in a molecule affects the interactions of the molecule with other polar groups and ions , electronegativities give information regarding relative chemical reactivity . For example , the relative electronegativities of carbon and silicon indicate that the hydrogen atoms in silicon hydrides should be considerably more negatively charged than those hydrocarbons.

Si  H  C H

1.8 2.1 2.5 2.1

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Chemical properties

Radii/pm: Si 4+ 26; atomic 117; covalent 117; van der Waals 200;
Si 4+271

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A brief introduction of silicon compound

Oxides of silicon:

There are two oxides of silicon , SiO and SiO2 .
Silicon monoxide is thought to be form by high temperature reduction of SiO2 with Si , but it's existence at room temperature is in doubt :
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                                          SiO2 + Si à 2SiO
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Silicon dioxide SiO2 ( silica ) is widely found as sand and quartz . Group 14 elements typically form 4 bonds . However , silicon cannot form pkV pkdouble bonds . Thus , SiO2 forms an infinite three-dimentional structure and SiO2 is a high melting solid . SiO2 exists in at least 12 different forms . The main one are quartz , tridymite and cristobalite , each of which has different structures at high and low temperatures . Pure SiO2 is colourless , but traces of other metals may colour it giving semi-precious gemstones such as amethyst ( violet) , rose quartz ( pink ) and non-precious material such as flint ( often black due to carbon ) .
The structure of silicon dioxide is as below :

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Each Si is tetrahedrally surrounded by four O atoms . Each corner is shared with another tetrahedron , thus give an infinite array .

Silicates

Silicate is a salt of silicic acid . In mineralogical chemistry the silicates include ; the unisilicates or orthosilicates ( fig i ) , salts of orthosilicic acid ; the bisilicates or metasilicates , salts of metasilicic acid ; the polysilicates or acid silicates , salts of the polysilicic acids ; the basic silicates or subsilicates , in which the equivalent of base is greater than would be required to neutralize the acid ; and the hydrous silicates , including the zeolites and many hydrated decomposition products .
The followings are the examples of structure of different silicates :
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( i ) structure of orthosilicates

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( ii ) structure of disilicates

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( iii ) structure of chain silicates

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( iv ) structure of  pyroxenes

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( v ) structure of sheet silicates

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( vi ) structure of cyclic silicates

                                             
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About 95% of earth crust is composed of silicate minerals , alumino-silicate clays or silica . These make up the bulk of all rocks , sands , and their breakdown products clays and soil . Many building materials are silicates : granite , slates , bricks and cement . Ceramics and glass are also silicates. The reasons for the high abundant of silicate in the earth is , during the cooling of the earth , the lighter silicate materials crystallzed and floated to the surface , resulting in the concentration of silicates in the earth crust .

Silicon tetrachloride

Silicon tetrachloride SiCl4 is volatile molecular compound . It is prepared by direct reaction of the elements :

Si + 2Cl2 à SiCl4

Much of the production of the tetrachloride is destined for purification by distillation (refer to the section : extraction of silicon) followed by reduction with hydrogen, magnesium or zinc to produce semiconductor-grade silicon :
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SiCl4 + 2Mg à Si + MgCl2

Or

SiCl4 + 2H2à Si +4HCl

Silicon halides are mild Lewis acids . They display this character when they add one or two ligands to yield complexes with a five-coordinate or six-coordinate central atom :

SiF4 + 2F- à [SiF6 ]2-

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