Question

The electronic configurations of chromium (at. no. 24) and copper (at. no. 29) are

• A. [Ar]4s\(^1\)3d\(^5\) and [Ar]4s\(^1\)3d\(^{10}\)
• B. [Ar]4s\(^2\)3d\(^4\) and [Ar]4s\(^1\)3d\(^{10}\)
• C. [Ar]4s\(^1\)3d\(^5\) and [Ar]4s\(^2\)3d\(^9\)
• D. [Ar]4s\(^2\)3d\(^4\) and [Ar]4s\(^2\)3d\(^9\)

Hint:

Remember the exceptions: Cr and Cu. Also, Mo (Z=42) and Ag (Z=47) follow a similar pattern.

Answer 1

The Correct Option is A

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\(^2\)3d\(^4\). To achieve a half-filled d-subshell (d\(^5\)), one electron from the 4s orbital is promoted to the 3d orbital. Actual configuration: [Ar]4s\(^1\)3d\(^5\). • Copper (Z=29): Expected configuration: [Ar]4s\(^2\)3d\(^9\). To achieve a fully-filled d-subshell (d\(^{10}\)), one electron from the 4s orbital is promoted to the 3d orbital. Actual configuration: [Ar]4s\(^1\)3d\(^{10}\).

Step 3: Final Answer:
The correct configurations are [Ar]4s\(^1\)3d\(^5\) for Cr and [Ar]4s\(^1\)3d\(^{10}\) for Cu, which corresponds to option (A).