D.

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.

A.

4

C.

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 O^2- ion is 8. Thus Na₂O has 4:8 coordination.