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Explain metal excess defects. |
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Answer» Solution : (i) Defects due to anionic vacancies : Alkali halides such as NaCl, KCL etc. show this type of defects. In this type of defects, the ANION is ABSENT from its lattice site and the same site is occupied by an electron. As a result of occupancy of an electron in place of anion, the electrical neutrality is maintained.  When a crystal of sodium chloride are heated in an atmosphere of sodium vapours, the sodium atoms are deposited on the surface of crystal and the `Cl^(-)` ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. The NaCl is formed by loss of electron by sodium atom to form Nations and the released electrons diffuse into the crystal and occupies anionic site. The crystal now has EXCESS of sodium. The anionic site occupied by unpaired electrons are called F-centres (from the Greek word Farbenzenter for colour centre). They impart yellow colour to the crystals of NaCl as a result of excitation of these electrons when they absorb energy from the visible light falling on crystals. Similarly excess lithium makes Lici pink and excess potassium makes KCl violet or lilac. (ii) Defects due to extra cations at interstitial sites : Zinc oxide (ZNO) is white in colour at a room temperature. Upon heating, it loses oxygen and turns yellow. `ZnO overset("Heating")(rarr) Zn^(2+) + 1/2O_2 + 2e^(-)` The crystal has now excess of zinc and its formula becomes `Zn_(1 + x)O`. The excess of `Zn^(2+)` ions thus formed moves to interstitial sites and electron to neighbouring interstitial sites. |
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