SEMICONDUCTORS

A semiconductor is a substance at has different resistances to the passages of an electric current under different circumstances. Semiconductors include the elements germanium,s and silicon, and the compounds lead sulphide, silicon carbide, cadmium sulphide, lead telluride, gallium arsenide and indium antimonide.

Semiconductors Conduct electricity Better than insulators, but not as well as good Conductors like metals. This gives them their Name, which means “half conductors. “How well they conduct electricity depends upon their Temperature. When a metal is heated, its resistance increases, when a semiconductor is heated its Resistance decreases.

Semiconductors are also sensitive to light. The greater the intensity of the light that Shines on them, the better they conduct electricity. The effects that light and heat energy have on Semiconductors make them extremely useful. They are used in photoelectric cells and in solar Batteries. The electrons of semiconductors do not carry electric current as readily as the electrons Of good electric conductors like metals. However, when the atoms of the material absorb heat or Light, the electrons become less tightly bound to their atoms. They can now conduct electricity.

Another special property of semiconductors is the way they behave when they are joined To another material, which may be a metal or a different semiconductor. The junction between the Different materials forms a boundary. It allows electricity to pass more properly and is used in Transistors. Transistors are much smaller and less complicated than old fashioned electronicTubes. They are used in radio, television, computers and calculators.

USES OF LEAD COMPOUNDS IN PAINTS

Varoius oxides of lead, basic lead carbonate and lead chromate are commonly used as Pigments in paints.

1. Lead Suboxide, Pb,O

It is black powder, obtained on heating plumbous oxalate in the absence of air Other than pigment, it is also used in the manufacture of lead storage batteries.When lead is heated in air at about 340°C, it absorbs oxygen and forms a bright scarlet Crystalline powder of red lead. Red lead is used for a variety of purposes.

2. Triplumbic Tetraoxide, (red lead, minium), Pb,O,

Existence of two forms, a rhombic (yellow) and a tetragonal (red). It is slightly soluble in water. It Is usually used in preparing flint glass and paints. Litharge is used in preparing oils and varnishes. If litharge is boiled with water and olive oil, lead oleate which is a sticky adhesive mass is formed And glycerine passes into solution.

When lead is heated in air at about 340°C, it absorbs oxygen and forms a bright scarlet Crystalline powder of red lead. Red lead is used for a variety of purposes.Ingredient in the manufacture of flint glass, matches and ceramic glazes.

3. Lead Dioxide, PbO,

When red lead is treated with concentrated nitric acid, it is decomposed into lead nitrate And lead dioxide. Lead dioxide is a reddish brown powder. It is not very soluble in water, but it does Dissolve in alkaline water to yield soluble plumbates. It is not affected by dilute acids.

4. White Lead

Basic lead carbonate 2PbCO,.Pb(OH), is an amorphous white pigment. It mixes readily With linseed oil and has a good covering power. If improperly prepared, it becomes crystalline And its covering power is reduced. White lead is not suitable for use as a good pigment since it is darkened by the hydrogen Sulphide which is frequently present in the atmosphere.

5. Lead Chromate (PbCrO₂)

It is used as a pigment under the name of chrome yellow. Orange or red basic lead Chromates are formed when lead chromate is boiled with dilute alkali hydroxide and are used as Pigments. The stable yellow modification of lead chromate is monoclinic. Mixture of lead Chromate with lead sulphate or barium sulphate are also used as yellow pigments.

A semiconductor is a substance at has different resistances to the passages of an electric current under different circumstances. Semiconductors include the elements germanium,s and silicon, and the compounds lead sulphide, silicon carbide, cadmium sulphide, lead telluride, gallium arsenide and indium antimonide.

 KEY POINTS

1. Boron occurs in traces and has been found to be important for the growth of many plants.
2. Borax (Na,B,O, 10H₂O), colemanite (Ca,B.O.SH,O), orthoboric acid (H,BO,) are the Common minerals of boron.
3. Aluminium, after oxygen and silicon, is the third most abundant element in the earth’s Crust. Feldspar (K₂O.Al₂O3.6SiO₂), corundum (Al₂O,), bauxite (Al,0,.2H₂O) and Cryolite (Na,AlF) are the common minerals of aluminium.
4. The pair of outermost electrons that does not readily take part in chemical combination is Termed as inert pair.
5. When heated, borax fuses, loses water of crystallization and swells up into a white Porous mass. It is employed in borax bead test for identification of coloured salts.
6. When a hot concentrated solution of borax is treated with a calculated quantity of conc.
7. H₂SO4, on cooling crystals of boric acid are produced.
8. When aluminium is burnt in oxygen a brilliant light is produced.
9. The electronic configuration of group IVA elements shows that they have four electrons In their valence shells, two of which are in s-orbital and the remaining two are in p-orbitals.
10. Feldspar, potash mica and zircon are the famous minerals of silicon.
11. Silica has a net work structure in which every silicon atom is bonded tetrahedrally to four Oxygen atoms and every oxygen atom is bonded to two silicon atoms.doid Sodium silicate, aluminium silicate, talc and asbestos are commercially important Compounds of silicon and oxygen called silicates.
12. Methyl silicones can be used as lubricants and for water proofing.
13. Semiconductors conduct electricity better than insulators. They are also light sensitive.
14. Oxides of lead are used as pigments.

EXERCISE

• Acid has the chemical formula HBO₂.

• Aluminium normally occurs as Portion of the earth.

• Aluminium gives That is not a metal.

• Minerals found in the rocks of outer Gas with hot concentrated H₂SO4.

• Boron forms The chemical formula of white lead is Bond in its compounds.

• Is the only element with less than four electrons in the outer most shell Is a semi-metal.

• In the Group IIIA of the periodic table .

• Borax that occurs as a natural deposit, is called…………

• Cryolite is an important mineral of aluminium and its formula is………..

• Is a substance that has different resistances to the passage of an……….

• Electric current under different circumstances.