The number of atoms in a face-centred cubic unit cell is:

• 4

• 6

• 1

• 2

Schottky defect generally appears in

• NaCl
• KCl
• CsCl 
• all the above

The presence of F-centres in a solid makes it

• conducting
• colourless
• coloured 
• non-conducting

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?

Write two important features, coordination numbr of the ions and number of formula units per unit cell for the following crystals:
(i) Cesium chloride, (ii) Zinc sulphide, (iii) Calcium fluorite, (iv) Sodium oxide.