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−K₂O.Al₂O₃.6SiO₂

(ii) Kaolinite−Al₂O₃.2SiO₂.2H₂O

(iii) Asbestos−CaO.3MgO.4SiO₂

Preparation

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

SiO₂(s) + 2C(s) →  Si(s) + 2CO(g)

(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.

SiCl₄(l) + 2H₂(g) →  Si(s) + 4HCl(g)

SiHCl₃(s) + H₂(g) →  Si(s) + 3HCl(g)

Physical Properties

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

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

(iii) Silicon exists in three isotopes, i.e. and but 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 1173 K to form silicon dioxide and with nitrogen of air at 1673 K to form silicon nitride.

Si(s) + O₂(g) SiO2(s)

Silicon dioxide

3Si(s) + 2N₂(g) Si₃N4(s)

Silicon nitride

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

Si(s) + 2H₂O(g) SiO₂(s) + 2H₂(g)

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

Si(s) + 2F₂(g) SiF₄(l)

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

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

Si(s) + C(s) 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.

Compounds of Silicon

Silicates: Details about silicates are given in the topic “Solid State”.

Silica

Occurrence

Silica or silicon dioxide occurs in nature in the free state as sand, quartz and flint and in the combined state as silicates like, Feldspar: K2O.Al2O3.6SiO2, Kaolinite: Al2O3.2SiO2.2H2O etc.

Properties

(i) Pure silica is colourless, but sand is usually coloured yellow or brown due to the presence of ferric oxide as an impurity.

(ii) Silicon dioxide is insoluble in water and all acids except hydrofluoric acid.

SiO₂(s) + 4HF(l) →  SiF₄(l) + 2H₂O(l)

(iii) It also combines with metallic oxides at high temperature giving silicates e.g.

SiO₂(s) + CaO(s) CaSiO₃(s)

(iv) When silica is heated strongly with metallic salts, silicates are formed and the volatile oxides are driven off as vapours.

SiO₂(s) + Na₂CO₃(s) →  Na₂SiO₃(s) + CO₂(g)

SiO₂(s) + Na₂SO₄(s) →  Na₂SiO₃(s) + SO₃(g)

3SiO₂(s) + Ca₃(PO₄)₂(s) →  3CaSiO₃(s) + P₂O₅(g)

The first two examples quoted here are important in glass making.

Uses

(i) Sand is used in large quantities to make mortar and cement.

(ii) Being transparent to ultraviolet light, large crystals of quartz are used for making lenses for optical instruments and for controlling the frequency of radio−transmitters.

(iii) Powdered quartz is used for making silica bricks.

(iv) Silica gel (SiO₂.xH₂O) is used as a desiccant (for absorbing moisture) and as an adsorbent in chromatography.