The Solid State

Chemistry I

Chemistry

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The density of chromium is 7.2 g cm^{–3}. If the unit cell is cubic with edge length of 289 pm, determine the type of the unit cell (Atomic mass of Cr = 52 amu).

Solution:

We have given that

Gram atomic mass of Cr(M) = 52.0 g mol^{-1}

Edge length of unit cell (a) = 289 pm

Density of unit cell $\left(\mathrm{\rho}\right)$ = 7.2 g cm^{-3 } Avogadro's Number $\left({\mathrm{N}}_{0}\right)=6.022\times {10}^{23}{\mathrm{mol}}^{-1}$

$\mathrm{\rho}=\frac{\mathrm{Z}\times \mathrm{M}}{{\mathrm{a}}^{3}\times {\mathrm{N}}_{\mathrm{A}}\times {10}^{-30}}$

or $\mathrm{Z}=\frac{\mathrm{\rho}\times {\mathrm{a}}^{3}\times {\mathrm{N}}_{\mathrm{A}}\times {10}^{-30}}{\mathrm{M}}$

$\mathrm{Z}=\frac{(7.2\mathrm{g}{\mathrm{cm}}^{-3})\times (289{)}^{3}\times (6.022\times {10}^{23}{\mathrm{mol}}^{-3})\times ({10}^{-30}{\mathrm{cm}}^{3})}{(52.0\mathrm{g}{\mathrm{mol}}^{-1})}=2$

Since the unit cell has 2 atoms, it is body centre in nature.

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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.

1237 Views

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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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.

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