Question

Electronic configuration of chromium is:

• A. [Ar] $3d^4\,4s^1$
• B. [Ar] $3d^4\,4s^2$
• C. [Ar] $3d^5\,4s^1$
• D. [Ar] $3d^5\,4s^2$

Hint:

Remember key exceptions in electronic configuration: Cr = [Ar] $3d^5\,4s^1$ Cu = [Ar] $3d^{10}\,4s^1$ They occur due to extra stability of half-filled and fully filled $d$ subshells.

Answer 1

The Correct Option is C

Concept: Electronic configuration follows the Aufbau principle, but some transition elements show exceptions due to extra stability of:

• Half-filled subshells ($d^5$)
• Completely filled subshells ($d^{10}$)

This stability arises from exchange energy and symmetrical electron distribution.
Step 1: Write expected configuration using Aufbau principle. Chromium has atomic number 24. Expected configuration: \[ \text{[Ar]} \, 3d^4\,4s^2 \]
Step 2: Apply stability of half-filled subshell. A half-filled $3d^5$ configuration is more stable than $3d^4$. So, one electron from $4s$ shifts to $3d$.
Step 3: Write actual configuration. \[ \text{[Ar]} \, 3d^5\,4s^1 \]