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

Step 1. Metal excess defect arises when a crystal contains MORE cations than anions -- seen in alkali halides such as NaCl and KCl. Electrical neutrality is restored either by anionic vacancies (equal in number to the excess cations) or by extra interstitial cation-electron pairs.

Step 2. Example: heating NaCl in sodium vapour deposits extra Na+ ions on the surface; Cl- ions diffuse out to combine with them, and the electron the sodium atom lost diffuses INTO the crystal to occupy the resulting anionic vacancy. Such a vacancy holding a trapped electron is called an F-centre, and gives the crystal (formula Na(1+x)Cl) a yellow colour.

Step 3. Metal deficiency defect arises when a crystal contains FEWER cations than anions -- only possible when the cation has variable oxidation states, so the remaining cations can be oxidised to compensate. …

Present the two defects side by side: which ion is in surplus/deficit, why it happens (variable oxidation sta …

• Reversing the two defect names -- 'excess' means MORE cations (metal-rich), 'deficiency' means FEWER cations (metal-poor); do not …