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:

Chromium ($3d^5 4s^1$) and Copper ($3d^{10} 4s^1$) are the two classic exceptions in the 3d series due to half-filled and fully-filled stability.

Answer 1

The Correct Option is C

Step 1: Atomic number of chromium.
Chromium has $Z = 24$ and belongs to the first row of d-block elements (Period 4).
Step 2: Expected configuration from Aufbau.
Strictly following the Aufbau principle, the expected configuration would be $[Ar]~3d^4~4s^2$. However, this is not observed experimentally.
Step 3: Extra stability of half-filled subshell.
A half-filled $d^5$ subshell has higher exchange energy and perfect spherical symmetry, making it exceptionally stable. One electron is therefore promoted from $4s$ to $3d$.
Step 4: Actual configuration.
The actual ground-state electronic configuration of chromium is $[Ar]~3d^5~4s^1$.
\[ \boxed{(C)} \]