Q.Answer the following question.
Explain the anomalous behavior of copper and chromium.

Step 1. Chromium (Z=24). Simple Aufbau filling predicts $[Ar]\,4s^2\,3d^4$, but a 3d subshell with 4 electrons is neither half-filled nor fully filled, so it is less stable. Interelectronic repulsion pushes one 4s electron into the empty 3d orbital, giving both subshells a half-filled arrangement: actual configuration $[Ar]\,4s^1\,3d^5$, which is more stable.

Step 2. Copper (Z=29). Simple filling predicts $[Ar]\,4s^2\,3d^9$, where 3d (9 electrons) is neither half- nor fully filled. One 4s electron shifts into 3d, completely filling it: actual configuration $[Ar]\,4s^1\,3d^{10}$, gaining extra stability from a fully-filled d subshell (with 4s left half-filled). …

Comparing the Aufbau-predicted vs actual configurations of Cr and Cu an …

• Writing chromium's actual configuration as 4s²3d⁴ (the naively 'expected', but wrong, configuration). …