247. Explain the reasons why:
(a) Frenkel defect is not found in pure alkali halides?
(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.