Question : Ionic solids , which have anionic vacancies due to metal excess defect , develop colour. Explain with the help of a suitable example ?

key points to answer this question : first recall which thing is responsible in solids for colors , so remember the Free unpaired electrons or we can say the F-centres are mainly responsible for the colour of solids. after that we will explain how F-centres are generated in a solids , we will discuss this thing by taking an example of NaCl  with excess of Na metal. in this way our answer will be completed.

solution : we take an example of sodium chloride (NaCl) crystals for explanation of metal excess defect. In this example NaCl crystals are heated up in an atmosphere of sodium vapour , the sodium (Na) atoms are deposited on the surface of the crystal after heating up. the Chloride ions (Cl ions) get diffused to the surface of crystal and these Cl ions combined with the sodium (Na) atoms and give NaCl. And this reaction happened by losing electron from the Na atom and create Sodium ion (Na+) .

The released electrons from the sodium (Na) atom occupy anionic sites by diffusing into the crystals. These released electrons from the sodium (Na) electrons absorb energy from visible light and emit radiations corresponding to yellow colour in the solid. These electrons which are responsible for color in solids are called F-centres .

F-centres word is taken from the German word Farbenzenter that’s meaning is ‘colour centre’

conclusion :  in this way the color occurred in solids. we have learned here development of colour in solids with Explanation the help of a suitable example.