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 describes the distribution of electrons in the atomic orbitals. For Chromium (\(\mathrm{Cr}\)), which has an atomic number of 24, the typical electron configuration can be perplexing due to an exception to the general order of filling orbitals.

Generally, electrons fill the orbitals in the order of increasing energy. According to the Aufbau principle, they should fill in the sequence from lower to higher energy levels, following the order: \(1s\), \(2s\), \(2p\), \(3s\), \(3p\), and so forth. The expected configuration for Chromium might appear as \(1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^4\,4s^2\), but Chromium's actual configuration deviates due to increased stability provided by a half-filled \(3d\) subshell.

For \( \mathrm{Cr} \), in order to achieve a more stable electron arrangement, one electron from the \(4s\) orbital is promoted to the \(3d\) orbital, resulting in a half-filled \(3d\) subshell and optimally filled \(4s\) orbital. This makes the configuration:

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

This configuration is due to the slightly lower energy of a half-filled \(d\) subshell compared to having a full \(4s\) orbital.

Thus, the correct electronic configuration of Chromium (\(\mathrm{Cr}\)) is:

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

Answer 2

The electronic configuration of an element describes the distribution of electrons among the atomic orbitals. For chromium (\(\mathrm{Cr}\)), which has an atomic number of 24, the electronic configuration needs to account for 24 electrons. Generally, electrons fill orbitals in a specific order based on increasing energy levels, known as the Aufbau principle, which typically suggests the following order: \(1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p,\) and so forth.

However, there is an exception for chromium due to enhanced stability provided by a half-filled \(3d\) subshell. Filling the orbitals goes as:

• \(1s\) can hold 2 electrons: \(1s^2\)
• \(2s\) holds 2 electrons: \(2s^2\)
• \(2p\) holds 6 electrons: \(2p^6\)
• \(3s\) holds 2 electrons: \(3s^2\)
• \(3p\) holds 6 electrons: \(3p^6\)
• Before filling the \(4s\) entirely, a special case occurs where electron configuration prefers half-filled \(3d\) for lower energy: filling the \(3d\) up to 5 electrons and \(4s\) with 1. Therefore, the remaining electrons are arranged with 5 in \(3d\): \(3d^5\)
• This leaves 1 electron for \(4s\), making it \(4s^1\)

Thus, chromium achieves enhanced stability by having a \(3d^5 4s^1\) configuration.

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

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