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Class 10 Class 12
Write two important features, coordination numbr of the ions and number of formula units per unit cell for the following crystals:
(i) Cesium chloride, (ii) Zinc sulphide, (iii) Calcium fluorite, (iv) Sodium oxide.

i) CsCl is simple cubic cell. Cesium ion is surrounded by eight chloride ion which are also disposed towards the corner of a cube therefore both type ions are in equivalent positions and the stoichiometry is 1:1 . the coordination of CsCl is 8:8.

ii) Zinc sulfide is a FCC unit cell. The net number of zinc cation per unit cell is four, and the net number of sulfide anions per unit cell is four therefore, the ratio of ZnS ion in the cell is 1:1.

iii)Calcium fluoride is a FCC unit cell. The net number of Calcium cation per unit cell is four and the net number of fluoride anion is eight. Therefore the ratio of CaF2 ion in cell is 1:2

iv) Na2O has the structure opposite to CaF2. In
this case coordination number of Na+ ions is 4 and that of O2- ion is 8. Thus Na2O has 4:8 coordination.

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 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.

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 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 .

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