What is the ground-state electronic configuration of copper (Z = 29)?
Correct answer A. [Ar]3d¹⁰4s¹
Explanation
Copper adopts 3d¹⁰4s¹ rather than 3d⁹4s² because a completely filled 3d subshell gives extra stability. This is one of the two common exceptions in the first transition series.
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