JEE Main 2021 · 25 February · morning · Single correct · +4 / −1

In which of the following pairs, the outer most electronic configuration will be the same?

  1. (a) Cr⁺ and Mn²⁺
  2. (b) Ni²⁺ and Cu⁺
  3. (c) V²⁺ and Cr⁺
  4. (d) Fe²⁺ and Co⁺
Answer and solution

Answer (a) Cr⁺ and Mn²⁺

Solution

Chemistry ma’am · 3 steps

Strip the 4s electrons first, then compare what is left in 3d for each ion.

  1. Removal rule. When a 3d metal forms a cation, it loses its 4s electrons before any 3d electrons. Write the neutral atom first, then take electrons away from 4s.
  2. Watch Cr and Cu. Chromium is [Ar] 3d⁵ 4s¹ and copper is [Ar] 3d¹⁰ 4s¹. Each has only one 4s electron, so the first electron removed empties 4s.
    IonNeutral atomIon configuration
    Cr⁺3d⁵ 4s¹3d⁵
    Mn²⁺3d⁵ 4s²3d⁵
    Ni²⁺ / Cu⁺3d⁸ 4s² / 3d¹⁰ 4s¹3d⁸ / 3d¹⁰
    V²⁺ / Cr⁺3d³ 4s² / 3d⁵ 4s¹3d³ / 3d⁵
    Fe²⁺ / Co⁺3d⁶ 4s² / 3d⁷ 4s²3d⁶ / 3d⁸
  3. So. Only (a) matches: Cr⁺ and Mn²⁺ both end in 3d⁵. Both ions have 23 electrons, so they are isoelectronic.

The idea to keep. For 3d cations, always empty 4s before touching 3d. Remember the two odd ones, Cr (3d⁵ 4s¹) and Cu (3d¹⁰ 4s¹), because they lose their single 4s electron first.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Atomic Structure

Asked about 170 times in JEE Main since 2020. This question is from 2021.