alleryThe electronic configuration of Cr (Z=24) is an exception, written as:
A[Ar] 4s² 3d⁴ (expected by Aufbau)
B[Ar] 4s² 3d²
C[Ar] 4s⁰ 3d⁶
D[Ar] 4s¹ 3d⁵ (half-filled stability)
Answer & Solution
Correct answer: D. [Ar] 4s¹ 3d⁵ (half-filled stability)
**Cr exception:** instead of [Ar] 4s² 3d⁴, prefers [Ar] 4s¹ 3d⁵.
Half-filled 3d⁵ is extra-stable (exchange energy + symmetric distribution).
**Cu similarly:** [Ar] 4s¹ 3d¹⁰ (full d).
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