Q.Write short note on metal excess and metal deficiency defect with an example.

Both are non-stoichiometric point defects -- here the crystal's cation:anion ratio genuinely shifts away from the ideal formula, unlike Schottky/Frenkel defects:

• Metal excess defect: MORE cations than anions. Seen in alkali halides (NaCl, KCl). Balanced either by anionic vacancies equal to the excess cations, or by extra interstitial cation-electron pairs. Classic case: NaCl heated in Na vapour -> Na+ ions deposit on the surface, Cl- diffuses out to meet them, and the electron the Na atom lost diffuses INTO the vacancy the departed Cl- left behind. Such a vacancy occupied by a trapped electron is called an F-centre, and gives the crystal (formula $Na_{1+x}Cl$) its yellow colour. ZnO similarly turns yellow on heating, as it loses O2 and generates interstitial Zn2+ and electrons. …