JEE Main 2021 · 22 July · evening · Single correct · +4 / −1
The set having ions which are coloured and paramagnetic both is :
- (a) Cu²⁺, Cr³⁺, Sc⁺
- (b) Cu²⁺, Zn²⁺, Mn⁴⁺
- (c) Sc³⁺, V⁵⁺, Ti⁴⁺
- (d) Ni²⁺, Mn⁷⁺, Hg²⁺
Answer and solution
Answer (a) Cu²⁺, Cr³⁺, Sc⁺
Solution
Chemistry ma’am · 3 steps
Write the d-electron count of every ion. One d0 or d10 ion knocks the whole set out.
- The test. An ion is paramagnetic when it has unpaired electrons. It is coloured when a d–d transition is possible, which needs a partly filled d subshell (d¹ to d⁹). A d⁰ or d¹⁰ ion is diamagnetic and colourless.
- Count d electrons. Remove electrons from 4s first, then from 3d. For example, Cu is [Ar] 3d¹⁰4s¹, so Cu²⁺ is 3d⁹. Sc is 3d¹ 4s², so Sc⁺ is 3d¹ 4s¹.
Option Ions and configuration Every ion coloured and paramagnetic? (a) Cu²⁺ 3d⁹, Cr³⁺ 3d³, Sc⁺ 3d¹ 4s¹ Yes: all have unpaired d electrons (b) Cu²⁺ 3d⁹, Zn²⁺ 3d¹⁰, Mn⁴⁺ 3d³ No: Zn²⁺ is d¹⁰ (c) Sc³⁺, V⁵⁺, Ti⁴⁺, all 3d⁰ No: none has a d electron (d) Ni²⁺ 3d⁸, Mn⁷⁺ 3d⁰, Hg²⁺ 5d¹⁰ No: Mn⁷⁺ is d⁰ and Hg²⁺ is d¹⁰ - So. Only set (a) has an incompletely filled d subshell in every ion, so all three are coloured and paramagnetic.
The idea to keep. Colour and paramagnetism in transition metal ions both come from unpaired d electrons. Find the d count: d⁰ and d¹⁰ ions are colourless and diamagnetic.
Answer from the NTA final answer key. Solution by PrepWiser.