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| 1. |
Ionic solids, which have anionic vacancies due to metal excess defect develop colour. Explain with the help of suitable example. |
| Answer» Solution :Taking the example of NaCl, when its crystals are heated in presence of sodium vapour some chloride ion leave their lattice sites to combine with sodium to form NaCl. For this reaction to occur Na atoms lose electrons to form `Na^(+)` ions. The electron that has been released diffuses into the crystal to occupy the ANION vacancies created by `Cl^(-)` ions. The crystal now has EXCESS of sodium. The sites occupied by unpaired electrons are called F-centres. They impart yellow colour to the crystal because they ABSORB ENERGY from the visible light and get EXCITED. | |