Electronic Configurations Of Elements And The Periodic Table

Step 1: Understanding the Concept:
Chromium (Cr) and Copper (Cu) are exceptions to the Aufbau principle due to the extra stability associated with half-filled and fully-filled d-subshells.

Step 2: Detailed Explanation:
• Chromium (Z=24): Expected configuration: [Ar]4s 3d . To achieve a half-filled d-subshell (d ), one electron from the 4s orbital is promoted to the 3d orbital. Actual configuration: [Ar]4s 3d . • Copper (Z=29): Expected configuration: [Ar]4s 3d . To achieve a fully-filled d-subshell (d ), one electron from the 4s orbital is promoted to the 3d orbital. Actual configuration: [Ar]4s 3d .

Step 3: Final Answer:
The correct configurations are [Ar]4s 3d for Cr and [Ar]4s 3d for Cu, which corresponds to option (A).