Question

F centre gives colour to Crystal due to whose presence

Answer 1

due to free electrons trapped in anion vacancies

Answer 2

Answer:

(1) The defect due to anion vacancies imparts colour to the colourless crystal.

(2) When a colourless crystal of NaCI is heated in the atmosphere of sodium vapour, the sodium atoms are deposited on the crystal surface.

(3) Due to diffusion of CI^- ions to the crystal surface vacancies are created at their regular sites.

(4) These diffused Cl^- ions combine with Na atoms on the surfaco forming NaCl along with relcasing electrons from sodium atoms.

Na+Cl^- ------> NaCl+e^-

The released electrons diffuse into the crystal and Occupy vacant sites of anions Cl^- in the crystal. The anion vacant sites occupied by electrons are called F-centres or colour centres. Due to colour centres NaCl shows yellow colour.

(6) Now NaCI crystal has excess of Na atoms having nonstoichiometric formmula Na_1+xCl10