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) 
.
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):

ALLOTROPES OF CARBON:

A characteristic property of the elements of carbon family is that these show allotropy. Example: carbon has two important allotropic forms i.e.  Crystalline and amorphous

(1) DIAMOND:

(1) Each carbon is linked to another atom and there is very closed packing in structure of Diamond.

(2) Density and hardness is very much greater for diamond because of closed packing in diamond due to sp3 hybrid and are tetrahedrally arranged around it. And C-C distance is 154 pm

(3) Diamond has sharp cutting edges that's why it is employed in cutting of glass.

(4) Diamond crystals are bad conductor of electricity because of absence of mobile electron.

(5) 1 carat of diamond = 200 mg.

(6) Diamond powder if consumed is fatal and causes death in minutes.

(2) GRAPHITE: 

(1) In Graphite Carbons are sp² hybridised out of the four valence electrons, three   involved in (sp²-sigma) covalent bonds form hexagonal layers and fourth unhybridised p– electron of each carbon forms an extended delocalized p-bonding with carbon atoms of adjacent layers

 (2) Each carbon is linked with 3 carbons and one carbon will be left and form a two dimensional shed like structure.

(3) Distance between two layers is very large so no regular bond is formed between two layers. The layers are attached with weak vander waal force of attraction.

(4) The carbon have unpaired electron so graphite is a good conductor of current.

(5) The C-C bond length within a layer is 141.5 pm while the inter layer distance is 335.4 pm shorter than that of Diamond (1.54 Å).

(6) Due to wide separation and weak interlayer bonds, graphite is sift , greasy and a lubricant character and low density.

(7) Graphite marks the paper black so it is called black lead or plumbago and so it is used in pencil lead.

(8) Composition of pencil lead is graphite plus clay .the percentage of lead in pencil is zero .

 (9) Graphite has high melting point so it is employed in manufacture of crucible.

(10) Graphite when heated with oxidizing agents like alkaline KMnO4 forms mellatic acid (Benzene hexa carboxylic acid).

(11) Graphite on oxidation with HNO₃ gives acid i.e. known as Graphite acid C₁₂H₆O₁₂

(3) FULLERENCES:

(1) A fascinating discovery was the synthesis of spherical carbon-cage molecules called fullerences. The discovery of fullerene was awarded the noble prize in chemistry (1996). Fullerenes   were first prepared by evaporation of graphite using laser.

(2) Fullerences are sooty material so formed consists of C₆₀ with small amount of C₇₀ and other fullerences containing an even number of carbon up to 350

(3) Fullerences have a smooth structure and unlike diamond and graphite, dissolved in organic solvent like toluene.

(4)  C₆₀ is the most stable fullerene. It has the shape of a football and called buckminsterfullerene

(5) C₆₀ consists of fused five and six membered carbon rings

(6) Six membered rings surrounded by alternatively by hexagons and pentagons of carbon.

(7) Five membered rings are surrounded by five hexagons carbon rings.

(8) There are 12 five –membered rings

(9) There are 20 Six –membered rings

(10) In fullerenes all the carbon sp2 hybridised each carbon formed three sigma bond and the fourth electron delocalized to formed pi bond .

(11) All the carbon atom are equivalent but all C-C bond are not equivalent.

(12)  In the structure C-C bonds of two different bond length occur at the fusion of two six membered rings the bond length is  C-C = 135.5 pm and at the fusion of  five and six membered rings C-C bond length is 146.7 pm.

(13) There are both single and double bonds

(14) The smallest fullerenes are C₂₀.

(15) Thermodynamically the most stable allotrope of carbon is considered to be graphite. This is due standard enthalpy of formation of graphite is taken zero .while enthalpy of formation of diamond and fullerenes are 1.90 KJ/Mole and 38.1 KJ/Mole respectively.