JEE Main 2025 · 4 April · evening · Single correct · +4 / −1

Consider the ground state of chromium atom (Z=24). How many electrons are with Azimuthal quantum number l=1 and l=2 respectively ?

  1. (a) 16 and 5
  2. (b) 12 and 5
  3. (c) 12 and 4
  4. (d) 16 and 4
Answer and solution

Answer (b) 12 and 5

Solution

Chemistry ma’am · 4 steps

Write chromium's real configuration with its half-filled 3d first, then count p and d electrons.

  1. Meaning of l. The azimuthal quantum number tells you the subshell. l = 1 means a p subshell and l = 2 means a d subshell.
  2. Chromium's configuration. Chromium is an exception to the Aufbau order. One 4s electron moves into 3d, because a half-filled 3d⁵ subshell is extra stable.

    Cr (Z=24): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

  3. Count each subshell. p electrons (l = 1): 2p⁶ + 3p⁶ = 12. d electrons (l = 2): only 3d⁵, so 5.
  4. So. There are 12 electrons with l = 1 and 5 with l = 2, which is option (b).

The idea to keep. For quantum-number counting, always use the actual configuration. Cr and Cu break the Aufbau order: Cr is 3d⁵ 4s¹ and Cu is 3d¹⁰ 4s¹.

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 2025.