Chemistry OCCURANCE, PREPARATION AND PROPERTIES OF SILICON
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OCCURANCE, PREPARATION AND PROPERTIES OF SILICON

Occurrence :

Silicon is the second most abundant `(27.2%)` element after oxygen `(45.5%)` in the earth's crust. It does not occur free in nature but in the combined state, it occurs widely in form of silica and silicates. All mineral rocks, clays and soils are built of silicates of magnesium, aluminium, potassium or iron. Aluminium silicate is however the most common constituent of rocks and clays. Silica is found in the free state in sand, flint and quartz and in the combined state as silicates like

(i) Feldspar - `K_2O*Al_2O_3*6SiO_2`

(ii) Kaolinite - `Al_2O_3*2SiO_2*2H_2O`

(iii) Asbestos - `CaO*3MgO*4SiO_2`

Preparation :

(i) From silica (sand) : Elemental silicon is obtained by the reduction of silica `(SiO_2)` with high purity coke in an electric furnace.

`SiO_2(s) + 2C(s) oversettext(high tempreture) rightarrow Si (s) + 2CO(g)`

(ii) From silicon tetrachloride `(SiCl_4)` or silicon chloroform `(SiHCl_3)` : 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.

`SiCl_4(l) + 2H_2(s) rightarrow Si(s) + 4HCl(g)`

`SiHCl_3(s)+ H_2(g) rightarrow Si(s) + 3HCl(g)`

Physical Properties :

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

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

(iii) Silicon exists in three isotopes, i.e. `text()_14^28Si` ,`text()_14^29Si` , and `text()_14^(30)Si`, but `text()_14^28Si` is the most common.


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 `1173` `K` to form silicon dioxide and with nitrogen of air at `1673` `K` to form silicon nitride.

`Si(s) + O_2(g) overset(1173K) rightarrow undersettext(Silicon dioxide)(SiO_2(s))`

`3Si(s) + 2N_2(g) overset(1673K) rightarrow undersettext(Silicon nitride)(Si_3N_4(s))`

Action of steam : It is slowly attacked by steam when heated to redness liberating dihydrogen gas.

`Si(s) + 2H_2O(g) oversettext(redness) rightarrow SiO_2(s) + 2H_2(g)`

(iii) Reaction with halogens : It bums spontaneously in fluorine gas at room temperature to form silicon tetrafluoride `(SiF_4)`.

`Si(s) + 2F_2(g) oversettext(room temperature) rightarrow SiF_4(l)`

(iv) Reaction with carbon : Silicon combines with carbon at `2500 ^oC` forming silicon carbide `(SiC)` known as carborundum.

`Si(s) + C(s) overset(2500^oC) rightarrow SiC(s)`

Carborundum is an extremely hard substance next only to diamond. It is mainly used as an abrasive and as a refractory material.

Uses :

(i) Silicon is added to steel as such or more usually in form of ferrosilicon (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 bronze and ferrosilicon.
 
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