Chemistry Transition Elements MDCAT 2019
PMDC Verified Question 43 of 68
Which of the following is the electronic configuration of Cr?
A
\( [Ar] \; 3d^{5} \; 4s^{2} \)
B
\( [Ar] \; 3d^{4} \; 4s^{2} \)
C
\( [Ar] \; 3d^{5} \; 4s^{1} \)
D
\( [Ar] \; 3d^{6} \; 4s^{0} \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: \( [Ar] \; 3d^{5} \; 4s^{1} \)
Concept:

Chromium (\(Cr\), \(Z=24\)) requires an anomalous electron configuration to maximize stability by achieving a half-filled \(d\)-subshell.

Solution:

  • The expected configuration based on straightforward filling is \([Ar] \; 3d^4 \; 4s^2\).


  • However, promoting one electron from the \(4s\) orbital to the \(3d\) orbital results in \([Ar] \; 3d^5 \; 4s^1\).


  • This yields five unpaired electrons in the \(3d\) subshell, generating perfectly symmetrical half-filled \(d\)-orbitals. This state minimizes electron repulsion and maximizes exchange energy, making it much more stable.


Why other options are incorrect:

  • Option B: The theoretically expected, but thermodynamically unstable form.


  • Option A: Matches Manganese (\(Z=25\)).

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