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Class 10 Class 12
Explain the reasons why:
(b) Zinc oxide appears yellow on heating?
(c) Solid containing F-centres are para magnetic?
(d) Uncharged atoms or molecules never crystallize in a simple cubic structure?
(e) A given element will have the same density in both a hexagonal close-packed structure and a cubic close-packed structure?

a) Frenkel defect is not found in the pure alkali metal halides because this defect is due to vacancy of ion ,which is shifted in interstitial spaces and because the ions are too large and cannot get into interstitial sites.

b) ZnO becomes yellow on heating due to frankel defect, when ZnO is heated, it loses oxygen reversibly and turns yellow in colour.

Zno + heat -->Zn2+ +1/O2 +2e-

The Zn2+ ions formed the vacant voids in the crystal to form non- stoichiometric solid. The released electrons are trapped in its neighborhood. It is therefore non-stoichiometric ZnO is yellow and shows increased conductivity.

c) The free electrons trapped in the anion vacancies are called as F-centers. Solids containing F-centres are paramagnetic because the electrons occupying the vacant sites are unpaired.

d) Uncharged molecules and atoms are packed more efficiently in closed-packet structures. Hence theses uncharged molecules and atoms do not crystallize in simple cubic structures.

e) In both the structure, the fraction of the total volume occupied is 0.74. The two structure have the same coordination number of 12.

599 Views

Why do solids have a definite volume?

A solid have a definite volume because the inter molecular distance  between its molecules are fixed.
1439 Views

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 .
1368 Views

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.
1055 Views

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.
2488 Views

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