alleryThe ground-state electronic configuration of Cu (Z = 29) is:
A[Ar] 3d⁹ 4s²
B[Ar] 3d¹⁰ 4s¹
C[Ar] 3d⁸ 4s²
D[Kr] 3d¹⁰
Answer & Solution
Correct answer: B. [Ar] 3d¹⁰ 4s¹
1. Naively, Cu (Z = 29) would be [Ar] 3d⁹ 4s².
2. But a fully-filled 3d¹⁰ subshell is extra-stable (exchange + symmetry).
3. So an electron promotes from 4s to 3d: actual config is [Ar] 3d¹⁰ 4s¹.
4. Same anomaly drives Cr: [Ar] 3d⁵ 4s¹ (half-filled stability).
_Source: syllabus Chem Ch 2 §2.6 Exceptional Configurations Cr/Cu_
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