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The Solid State

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Chemistry I

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Class 10 Class 12
Pure silicon is an insulator. Silicon doped with phosphorus is a semi-conductor. Silicon doped with gallium is also a semiconductor. What is the difference between the two doped silicon semi-conductor?

Phosphorus has one excess valence electron (compared with Si) after forming the four covalent bonds normally with silicon. This excess electron gives rise to electronic conduction. That is why silicon becomes semi-conductor on doping with phosphorus. It is called n-type semi-conductor.

Gallium has only three valence electrons. It creates an electron deficient bond or a hole when it is doped with silicon. Such holes can move in the crystal giving rise to electrical conductivity. Thus silicon doped with gallium is also semi-conductor due to movement of holes. It is called-p-type semiconductor.


Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.

Amorphous solids: Polyurethane, teflon, cellophane, polyvinyal chloride, fibre glass.
Crystalline solids: naphthlene, benzoic acid, potassium nitrate, copper.

Why is glass considred a super-cooled liquid?

Glass is amorphous solids and  have a tendency to flow, though very slowly. Therefore, these are called pseudo solids or super cooled liquids .

Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show clevage property?

Refractive index of a solid which have the same value along all directions are isotropic in nature. It would not show cleavage property.

Why do solids have a definite volume?

A solid have a definite volume because the inter molecular distance  between its molecules are fixed.

Why are solids rigid?

Solids are rigid because the internuclear distance between the  molecules of a solid are very less and closely packed and their positions are fixed due to the strong forces of attraction between them.