Question

Electronic configuration of chromium is :

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

Hint:

The two most common exceptions in the 3d series are: 1. Chromium (Z=24): \(3d^5 \, 4s^1\) (Half-filled) 2. Copper (Z=29): \(3d^{10} \, 4s^1\) (Fully-filled)

Answer 1

The Correct Option is C

Concept: The electronic configuration follows the Aufbau principle, Pauli's exclusion principle, and Hund's rule. However, certain elements show anomalies due to the extra stability associated with half-filled or fully-filled subshells. Step 1: Determining the atomic number.
Chromium (Cr) has an atomic number of \(Z = 24\).

Step 2: Applying the Aufbau principle.
Expected configuration based on energy levels: \[ 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^4 \] Using the noble gas Argon (\(Z=18\)) as a core: Expected: \( [\text{Ar}] \, 4s^2 \, 3d^4 \).

Step 3: Accounting for stability exceptions.
In the case of Chromium, an electron from the \(4s\) orbital shifts to the \(3d\) orbital. This results in a \(3d^5 \, 4s^1\) configuration.

• Why? The \(3d^5\) configuration is exactly half-filled.

• Result: Half-filled subshells have increased stability due to the symmetrical distribution of electrons and high exchange energy.
Therefore, the correct configuration is \( [\text{Ar}] \, 3d^5 \, 4s^1 \).