Question

Which of the following is the correct electronic configuration of \( \mathrm{Cr} \) (Chromium, atomic number 24)?

• A. \( 1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^4\,4s^2 \)
• B. \( 1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^5\,4s^1 \)
• C. \( 1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^6\,4s^0 \)
• D. \( 1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^3\,4s^3 \)

Hint:

Chromium exhibits exceptional stability with half-filled \( 3d \) subshell.

Answer 1

The Correct Option is B

The electronic configuration of an element details electron distribution across atomic orbitals. For Chromium (\(\mathrm{Cr}\)), with an atomic number of 24, its typical electron configuration is exceptional, deviating from the general orbital filling order.

Electrons typically occupy orbitals in order of increasing energy, following the Aufbau principle: \(1s\), \(2s\), \(2p\), \(3s\), \(3p\), and so on. The predicted configuration for Chromium is \(1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^4\,4s^2\). However, Chromium's actual configuration differs because a half-filled \(3d\) subshell offers greater stability.

To achieve a more stable electron arrangement for \( \mathrm{Cr} \), one electron moves from the \(4s\) orbital to the \(3d\) orbital. This results in a half-filled \(3d\) subshell and an optimally filled \(4s\) orbital, yielding the configuration:

\(1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^5\,4s^1\)

This configuration is favored because a half-filled \(d\) subshell has slightly lower energy than a full \(4s\) orbital.

Therefore, the correct electronic configuration for Chromium (\(\mathrm{Cr}\)) is:

\(1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^5\,4s^1\)