JEE Main 2022 · 25 June · morning · Single correct · +4 / −1
The pair, in which ions are isoelectronic with AI³⁺ is :
- (a) Br⁻ and Be²⁺
- (b) Cl⁻ and Li⁺
- (c) S²⁻ and K⁺
- (d) O²⁻ and Mg²⁺
Answer and solution
Answer (d) O²⁻ and Mg²⁺
Solution
Chemistry ma’am · 3 steps
Isoelectronic means the same number of electrons. So count electrons, not protons, for every ion.
- Electrons in an ion. Start from the atomic number Z. Subtract the charge for a cation and add it for an anion: electrons = Z - (charge).
- Target ion. Aluminium has Z = 13, so Al³⁺ has 13 - 3 = 10 electrons. That is the neon configuration, 1s² 2s² 2p⁶.
Option First ion Second ion Both equal 10? (a) Br⁻: 35 + 1 = 36 Be²⁺: 4 - 2 = 2 No (b) Cl⁻: 17 + 1 = 18 Li⁺: 3 - 1 = 2 No (c) S²⁻: 16 + 2 = 18 K⁺: 19 - 1 = 18 No, both have 18 (d) O²⁻: 8 + 2 = 10 Mg²⁺: 12 - 2 = 10 Yes - So. Only (d) has both ions with 10 electrons. O²⁻ and Mg²⁺ are isoelectronic with Al³⁺.
The idea to keep. For isoelectronic questions, find the noble gas each ion copies. N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ all have 10 electrons, like neon.
Answer from the NTA final answer key. Solution by PrepWiser.