The Solid State

This is due to F centre, electron become exicted and when they come to ground state they radiate energy in visible reigon.

Schottky defect decrease the density solid where as no such defect arise due to frenkel defect.

The maximum possible coordination number of an atom in hcp structure of an element is 12.

Body centre wholly belongs to the unit cell in which it is present. Thus
in a body-centered cubic (bcc) unit cell:
(i) 8 corners × 1/8per corner atom= 8 x 1/8 = 1 atom
(ii) 1 body centre atom = 1 × 1 = 1 atom
∴ Total number of atoms per unit cell = 2 atoms
For face centred 

(i) 8 corners atoms × 1/8 atom per unit cell
= 8  x1/8 = 1 atom

(ii) 6 face-centred atoms ×1/2 atom per unit cell = 6 ×1/2 = 3 atoms
∴ Total number of atoms per unit cell = 4 atoms