Silicon and Its Compounds:

(1) SILICON (Si): 

(1) The Silicon was discovered by Berzelius in 1824.

(2) The silicon name is taken from Latin silver which means “flint”.

(3) Silicon is the second most abundant (27.2%) element after oxygen (45.5%) in the earth's crust.

(4) Silicon is a crystalline semi – metal or metalloid. One of its forms is shiny, grey and very brittle. In another allotropic form silicon is a brown amorphous powder most familiar in “dirty” beach sand.

(5) Silicon does not occur free in nature but in the combined state, it occurs widely in form of silica (SiO₂) and silicates (SiO₄ ^-4) .

 (6) All mineral rocks, clays and soils are built of silicates of magnesium, aluminium, potassium or iron.

(7) Aluminium silicate is however the most common constituent of rocks and clays.

(8) the most common compound of silicon is Silica and Silica (SiO₂) is found in the Free State in sand, flint and quartz and in the combined state as silicates like

(i) Feldspar   K₂O.Al2O₃. 6SiO₂

(ii) Kaolinite   Al₂O₃. 2SiO₂. 2H₂O

(iii) Asbestos CaO. 3MgO. 4SiO₂

(8) Silica (SiO_­2) is the most abundant chemical compound in the earth’s crust.

PREPARATION SILICON:

(i) From silica (sand): Elemental silicon is obtained by the reduction of silica (SiO2) with high purity coke in an electric furnace

(ii) From silicon tetrachloride (SiCl4) or silicon chloroform (SiHCl3): Silicon of very high purity required for making semiconductors is obtained by reduction of highly purified silicon tetrachloride or silicon chloroform with dihydrogen followed by purification by zone refining.

PHYSICAL PROPERTIES:

(i) Elemental silicon is very hard having diamond like structure.

(ii) It has shining luster with a melting point of 1793K and boiling point of about 3550K.

(iii) Silicon exists in three isotopes, i.e. ₂₈Si¹⁴, ₂₉Si¹⁴ and ₃₀Si¹⁴ but ₂₈Si¹⁴ is the most common isotope.

CHEMICAL PROPERTIES:

Silicon is particularly unreactive at room temperature towards most of the elements except fluorine.

Some important chemical reactions of silicon are discussed below.

(i) Action of air: Silicon reacts with oxygen of air at 1173K to form silicon dioxide and with nitrogen of air at 1673K to form silicon nitride,.

(ii) Action of steam: It is slowly attacked by steam when heated to redness liberating hydrogen gas.

(iii) Reaction with halogens: It burns spontaneously in fluorine gas at room temperature to form silicon tetra fluoride (SiF4).

However, with other halogens, it combines at high temperatures forming tetra halides.

(iv) Reaction with carbon: Silicon combines with carbon at 2500 °C forming silicon carbide (SiC) known as carborundum. Carborundum is an extremely hard substance next only to diamond. It is mainly used as an abrasive and as a refractory material.

USES OF SILICON:

(i) Silicon is added to steel as such or more usually in form of ferro silicon (an alloy of Fe and Si) to make it acid-resistant.

(ii) High purity silicon is used as semiconductors in electronic devices such as transistors.

(iii) It is used in the preparation of alloys such as silicon-bronze, magnesium silicon.

(2) SILICA (SiO2):

Silicon is unable to form pp - pp bond with oxygen atom due to its relatively large size. Thus it satisfies its all four valency with four oxygen atoms and constitutes three - dimensional network. In this structure each oxygen atom is shared by two silicon atoms. Three crystalline modification of SiO₂ are quartz, cristobalite and tridymite of which quartz and cristobalite are important.

Quartz (rock crystal) is the purest form of silica. It is used in preparation of costly glasses and lenses. It is also used as piezoelectric material (crystal oscillators and transducers).

Several amorphous forms of silica such as silica gel and fumed silica are known. Silica gel in made by acidification of sodium silicate and when dehydrates, is extensively used as a drying agent in chromatographic and catalyst support.

(3) Silicon Carbide (SiC): Carborundum:

Silicon carbide (SiC) is a compound of silicon and carbon. It is extremely rare on Earth in mineral form (moissanite) and it has semiconductor properties. It has a bluish-black appearance. It has a large number of crystalline forms.Silicon Carbide (SiC): Carborundum:

(4) SILICATES:

(1) Silicates is the general term applied for the solids with silicon – oxygen bonds.

(2) Silicates are regarded as the salts of silicic acid, H₄SiO₄.All the silicates are comprised of SiO₄ units.

(3) The Silicates units have a tetrahedral structure formed as a result of sp3 hybridization. Silicon atom has its complete octet but each oxygen atom is still short of one electron to complete its octet. They can complete their octet by taking up 4 electrons from a metal, getting converted to an anion [SiO4]^-4

(4) All the solids Silicates contain silicate ion (SiO₄)^4- as the basic structural unit.

(5)The silicate ion is tetrahedral in structure and when the one or more oxygen atoms between such tetrahedrons, a complex structure arise.

(6) All the silicates are non planer.

 CLASSIFACATION OF SILICATES:

The silicates may be classified in to following groups chain silicates, ring silicates, cyclic silicates, sheet silicates, three – dimensional silicates depends on the way in which the (SiO₄)^4- tetrahedral units are linked together

(1) Ortho silicates or Neso Silicates

(2) Pyro silicates or Sorosilicates or Disisilicates  

(3) Meta silicates (A) Cyclic meta silicates (B) Linear chain Meta Silicates

(4) Double chain Silicates or Amphiboles

(5) Sheet Silicates or phyllo silicates

(6) 3D-Silicates

(5) SILICONES (R2SiO WITH -Si-O-Si- LINKAGE):

(1) Silicones are organosilicon polymeric compounds containing repeated R₂SiO units and (-Si-O-Si-) linkage.

(2) The name is given silicone because their empirical formula is analogous to that of ketones (R₂CO).

(3)  The Silicones are form by hydrolysis of silicone tetra chloride (SiCl₄) 
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USES OF SILICONES:

(1) Silicones are chemically inert, water repelling nature, heat resistance and having good electrical insulating properties.

(2) Silicones are used as sealants, greases, electrical insulators and for water proofing of fabrics
                           SILICONES: (R2SiO WITH -Si-O-Si- LINKAGE):