Electronic configuration, Aufbau, Pauli, Hund

Why this is asked: Cr ([Ar]3d⁵4s¹) and Cu ([Ar]3d¹⁰4s¹) break the Aufbau filling order for the extra stability of a half-filled or filled d-subshell — NEET's standard exceptions. And when a transition metal ion forms, the 4s electrons are removed first, not 3d, even though 4s filled first — write Fe²⁺ with a 3d⁴4s² tail instead of the correct 3d⁶ and the trap has worked.

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