JEE Main 2021 · 26 August · morning · Single correct · +4 / −1
Given below are two statements.
Statement I : Frenkel defects are vacancy as well as interstitial defects.
Statement II : Frenkel defect leads to colour in ionic solids due to presence of F-centres.
Choose the most appropriate answer for the statements from the options given below :
- (a) Statement I is false but Statement II is true
- (b) Both Statement I and Statement II are true
- (c) Statement I is true but Statement II is false
- (d) Both Statement I and Statement II are false
Answer and solution
Answer (c) Statement I is true but Statement II is false
Solution
Chemistry ma’am · 4 steps
In Frenkel the ion only moves house inside the crystal, so no electron gets trapped and no colour appears.
- What Frenkel does. A smaller ion, usually the cation, leaves its lattice site and squeezes into an interstitial space. One vacancy and one interstitial ion form together, so it is both a vacancy and an interstitial defect. Statement I is true.
- What an F-centre is. An F-centre is an anion vacancy holding a trapped electron. It forms in metal excess defects, for example when NaCl is heated in sodium vapour, and that trapped electron absorbs visible light to give colour.
- Frenkel has none. In a Frenkel defect no ion leaves the crystal and no extra electron enters it. The solid stays stoichiometric with no trapped electrons, so there are no F-centres and no colour. Statement II is false.
- So. Statement I is true and Statement II is false, which is option (c).
The idea to keep. Frenkel and Schottky are stoichiometric defects and do not colour a crystal. Colour from F-centres needs extra electrons trapped in anion vacancies, which happens only in metal excess defects.
Answer from the NTA final answer key. Solution by PrepWiser.