Nonmetal silicon properties extraction uses compounds for std 8 to 12

Earths & Elements

Revision

Silicon

Occurrence
Extraction
Physical properties
Chemical properties
Silicon and electronics
Some uses of silicon compounds.


(1) Nonmetals occur in solid, liquid and gaseous states.
(2)Nonmetals are chemically active.
(3)All nonmetals, except graphite, are bad conductors of electricity.
(4) Carbon, Sulphur and phosphorous which are nonmetals, exhibit allotrophy.

INTRODUCTION
Compounds of silicon in the form of sand, clay, rocks, etc: have been used since very early times. The word silicon is derived from Latin word which means stone or flint. Silicon is used in making semiconductor devices, such as diodes and transistors. The atomic number of silicon is 14 an its mass number is 28 its symbol is Si.

Earths Elements for std 8 to 12

Remember:
Silicon was first isolated in 1824 and was found to be an element by John Berzelius, a Swedish chemist.

Activity:
Collect different forms of silica materials. Try to identify the difference between them. 

OCCURANCE
Silicon does not occur in free state . In the combined form it ranks next only to oxygen in abudance. Owing to its affinity to oxygen, silicon occurs as silica or silicates. Silica is silicon dioxide. Silicate is a compound of silica and metallic oxides.

Silicon stone 

Silicate is a major constituent of igneous rocks. Aluminium silicate is the most widely distributed silicate which is the constituent of clay. Sodium aluminium silicate is called zeolite. Like carbon silicon also exhibits allotropy. Silicon is the chief constituent of semi precious stones like gamet, zircon, topaz and opal.

EXTRACTION
Method 1: 

Silicon exists in two allotropic forms- amorphous and crystalline. Amorphous silicon is extracted from quartz.
Finely powdered silica (Sand and quartz) is mixed with magnesium powder and heated in a fire-clay crucible. Magnesium oxide and silicon are formed. The product is washed with dilute hydrochloric acid to dissolve magnesium oxide. Then it is washed with hydrofluoric acid to remove unchanged silica. The powder left behind is silicon in its amorphous form.

Reaction of Silicon with Magnesiou

Method 2:
Less pure silicon is obtained by reducing silica with coke. When excess of silica is heated with coke, pale yellow coloured silicon is obtained in its crystalline form. 

Reaction of Silica with Coke.

PROPERTIES OF SILICAON
Physical properties : 

Amorphous silicon is a dark brown powder insoluble in water. Crystalline silicon forms pale yellow crystals isomorphous with diamond its crystals re hard enough to scratch glass. Its melting point is 1683K and boiling point is 2628K.
Even though silicon is a nonmetal, it is a semiconductor.

Chemical Properties :
Amorphous silicon is more active than the crystalline form. 
It burns brilliantly in air and vigorously in oxygen to form silicon dioxide.

Reaction of  Silicon  with air

Silicon does not react with water. But red hot silicon decomposes steam liberating hydrogen. 

Reaction Silicon does not reacts with water

When a mixture of silicon and coke is heated in an electric furnace to about 3073K, silicon carbide is formed. 

Reaction of Silicon with Coke

SILICON AND ELECTRONICS
Silicon is an intrinsic semiconductor. It behaves as and insulator at absolute zero temperature, because all the electrons are bound to the atoms. Let us see what happens at the room temperature. Room temperature of about 300 K may be sufficient to make a valence electron of a silicon atom to move away from the influence of its nucleus. Thus covalent bond is broken. When this happens the electron becomes delocalized. This is how silicon becomes an intrinsic semi conductor.
The conductivity of silicon can be increased by a process called doping. You have studied about doping in the chapter on electronics.
Silicon has 4 electrons in the otermost orbit of its atom. Doping with a 15th group element like phosphorus provides 5 electrons. Foour of these electrons form covalent bonds with the four neighbouring silicon atoms. The 5th electron is free and enhances the electrical conductivity of silicon. Thus the addition of 15th group element makes silicon a n-type semiconductor.
When the 13th group element like boron is added to silicon it provides 3 electrons. These 3 electrons form covalent bonds with 3 electrons of 4 neighbouring silicon atoms. The remaining 4th silicon atom is unable to form a covalent bond due to shortage of an electron. Such a shortage of an electron is treated as positive charge and considered as a hole. Thus the 13th group element makes silicon a p-type semiconductor.

Activity:
Adding a pentavalent of a trivalent element to a tetravalent element leading to the formation of an extrinsic semiconductor is known as doping.

USES OF SILICON COMPOUNDS
Silicon compounds are widely used in various fields. Some of them are given here. 

Uses of Silicon in Household

(1) Quartz glass is used for the manufacture of threads which are highly elastic which are used in electrical instruments ( mirror galvanometers). 

Uses of Silicon Quartz

Uses of Silicon Quartz

(2) Quartz is used in making chemical apparatus and optical instruments. 

Silicon Powder 

Silicon powder use for glass & Porcelain

(3) Sand is used in the manufacture of glass and porcelain. 

Sand and stone for Building Material

(4) Sand and stone are used as building materials. 

Silicon uses in Calico printing

(5) Sodium silicate is called water glass. Chemically water glass is sodium silicate with excess of silica. It is used in calico printing. 

Silicon Abrasive for Grinding glass

Silicon Abrasive for Grinding glass

(6) Silicon carbide (SIC) is used as an abrasive for grinding glass. 

Silicon use as Insulating material

Silicon use as Electrical Appliances

(7) Silicones are excellent insulating material for electric motors and other electrical appliances. 

Silicon used in softening hard water

(8) Sodium aluminium silicates are used in softening hard water. 

Activity:

Silicones are rubbery compounds of silicon, oxygen and hydrocarbons. Their molecular structures are similar to those of long chain hydrocarbons.

POINTS TO REMEMBER

(1)Electronic configuration of silicon is 1s2, 2s22p6, 3s23p3. 

(2) Silicon is a tetra valent element. 

(3) The compounds of silicon are sand, clay and rocks. 

(4) Silicon occurs in nature in the form of silicon oxides and silicates. 

(5) The allotropic forms of silicon are amorphous and crystalline. 

(6) Silicon is manufactured by heating silica with magnesium powder. 

(7) Though silicon is a nonmetal, it is a semiconductor. 

(8) The covalent bonds of silicon are broken at 300K and electrons are delocalized which makes silicon an intrinsic semi conductor. 

(9) When silicon is doped with 15th group elements like phosphorus, silicon becomes n-type semi conductor. 

(10) When silicon is doped with 13th group elements like boron, silicon becomes p-type semi conductor. 

(11) Silicon carbide is used as an abrasive for grinding glass. 

(12) Silicon is used in the manufacture of glass and porcelain. 

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