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Class 10 Class 12
An element crystallizes in a structure having a fcc unit cell of an edge 200 pm. Calculate its density if 200 g of this element contains 24 x 1023 atoms.

Solution:
We have given that
Edge length of the unit cell
= 200 pm = 200 x 10–10 cm

Vol. of the unit cell
= (200 x 10–10 cm)= 8 x 10–24 cm3
Therefore, volume of the substance
= Vol. of unit cell x Vol. of 1 unit cell.

Since the element has a fcc unit cell, number of atoms per unit cell = 4.
Total number of atoms = Atoms/unit cell x number of unit cells.
= 24 x 1023atoms = 4

atoms/unit cell x No. of unit cells.

∴   No. of unit cells =

∴  Volume of the substance

Now,

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

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