Question

What is the electronic configuration of a chromium atom (Z = 24)? Give a reason for your answer.

Answer 1

1. Electronic configuration (Cr, Z = 24): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹; shorthand electronic configuration is [Ar] 3d⁵ 4s¹.
2. Aufbau expectation: the simple filling order would give [Ar] 3d⁴ 4s², but chromium actually adopts [Ar] 3d⁵ 4s¹.
3. A half-filled 3d subshell (d⁵) is extra stable. One 4s electron is promoted into 3d to maximize exchange energy and reduce electron repulsion, so the atom is more stable with [Ar] 3d⁵ 4s¹.