Question

The outer electronic configuration of ground state chromium is

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

Hint:

Remember the exceptions: Cr and Cu. Cr is \(3d^5 4s^1\), Cu is \(3d^{10} 4s^1\).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept
This question tests the knowledge of electronic configurations of d-block elements, specifically the exceptions to the Aufbau principle. The Aufbau principle states that electrons fill atomic orbitals in order of increasing energy. However, there are exceptions due to the enhanced stability of half-filled and completely filled subshells.
Step 2: Detailed Explanation
Chromium (Cr) has an atomic number (Z) of 24.
According to the Aufbau principle, the electronic configuration should be filled in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d. This would lead to the expected configuration:
\(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^4\), or in shorthand, \([\text{Ar}] 4s^2 3d^4\).
However, subshells that are exactly half-filled (like d⁵) or completely filled (like d¹⁰) are more stable than other configurations. This stability arises from two factors: symmetrical distribution of electrons and higher exchange energy.
In the case of chromium, the energy difference between the 4s and 3d orbitals is very small. To achieve the greater stability of a half-filled d-subshell, one electron from the 4s orbital promotes to the 3d orbital.
This results in the configuration:
\( [\text{Ar}] 4s^1 3d^5 \)
In this configuration, both the 4s and 3d subshells are half-filled, which is a state of high stability.
The outer electronic configuration is therefore written as 3d⁵ 4s¹.
Step 3: Final Answer
The correct outer electronic configuration of ground state chromium is 3d⁵ 4s¹.