(i) Write any two differences between Schottky defect and Frenkel defect.

Question

Schottky and Erenkel defects are stoichiometric defects. 

(i) Write any two differences between Schottky defect and Frenkel defect.

(ii) When pure NaCI (Sodium Chloride) crystal is heated in an atmosphere of sodium vapours, it turns yellow. Give reason.

Answer 1

(i)

Schottky defect	Frenket defect
1. Vacancy defect which arises due to the messing of equal number of cations and anions form the lattice sites.	Interstitial defector dislocation defect which arises when the smaller ion, usually cation is dislocated from its normal site to an interstitial site.
2. Cation and anion in are of almost similar sizes	There is a large difference size of ions
3. The density of the crystal is lowered	It does not affect the density of the crystal

(ii) It is caused by metal excess defect due to anion vacancies. When crystals of NaCI are heated in a atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl- ions diffuse to the surface of the crystal and combine with Na atoms to give NaCI. The electrons released from Na atoms diffuse into the crystal and occupy anionic sites to form Fcentres, which impart yellow colourto the crystal. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.