JEE Main 2020 · 5 September · morning · Single correct · +4 / −1
The correct electronic configuration and spin-only
magnetic moment (BM) of Gd³⁺ (Z = 64), respectively, are :
- (a) [Xe]5f⁷ and 8.9
- (b) [Xe]4f⁷ and 7.9
- (c) [Xe]5f⁷ and 7.9
- (d) [Xe]4f⁷ and 8.9
Answer and solution
Answer (b) [Xe]4f⁷ and 7.9
Solution
Chemistry ma’am · 4 steps
Gd is a lanthanide, so its f-electrons go into 4f. Remove 6s first, then 5d, and count what is left.
- Neutral atom. Gadolinium (Z = 64) is [Xe] 4f⁷ 5d¹ 6s². The half-filled 4f⁷ is extra stable, so the next electron goes into 5d instead of making 4f⁸.
- Form the ion. To make Gd³⁺, take two electrons from 6s and one from 5d. That leaves [Xe] 4f⁷. It is 4f, not 5f, because 5f fills only in the actinides.
- Unpaired electrons. Seven f orbitals hold seven electrons, one in each, all with parallel spins. So n = 7 unpaired electrons.
μ = √n(n+2) = √(7 × 9) = √63 ≈ 7.9 BM
- So. Gd³⁺ is [Xe] 4f⁷ with a spin-only moment of about 7.9 BM, which is option (b).
The idea to keep. For lanthanide ions, take electrons from 6s and 5d before 4f, then count unpaired electrons and use √n(n+2). Lanthanides fill 4f, actinides fill 5f.
Answer from the NTA final answer key. Solution by PrepWiser.