Question

Ionic solids which have anionic vacancyes due to metal excess defect develop color explain with example

Answer 1

Its bcoz of presence of f-centre
eg NaCl gives yellow colour on heated in an atm of sodium vapours

Answer 2

Answer:

• The colour develops because of the presence of electrons in the anionic sites.
• These electrons absorb energy from the visible part of radiation and get excited.  
• For example, when crystals of NaCl are heated in an atmosphere of sodium vapours, the sodium atoms get deposited on the surface of the crystal and the chloride ions from the crystal diffuse to the surface to form NaCl with the deposited Na atoms.
• During this process, the Na atoms on the surface lose electrons to form Na+ ions and the released electrons diffuse into the crystal to occupy the vacant anionic sites.
• These electrons get excited by absorbing energy from the visible light and impart yellow colour to the crystals.