Chemistry Transition Elements PMDC 2020
PMDC Verified Question 41 of 68
Electronic configuration of chromium (proton number 24) is:
A
\( [Ar] \; 3d^{4} \; 4s^{2} \)
B
\( [Ar] \; 3d^{5} \; 4s^{2} \)
C
\( [Ar] \; 3d^{5} \; 4s^{1} \)
D
\( [Ar] \; 3d^{5} \; 4s^{0} \)
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 (\(Z=24\)) exhibits an anomalous electron configuration. It shifts an electron from the \(4s\) subshell to the \(3d\) subshell to gain thermodynamic stability.

Solution:

  • The theoretically expected configuration by strict Aufbau rules would be \([Ar] \; 3d^4 \; 4s^2\).


  • However, a half-filled \(d\)-subshell (\(d^5\)) is highly symmetrical and exchange energy is maximized, making it exceptionally stable.


  • Therefore, one \(4s\) electron is promoted: \(4s^2 \rightarrow 4s^1\) and \(3d^4 \rightarrow 3d^5\).


  • The true ground state configuration is \([Ar] \; 3d^5 \; 4s^1\).


Why other options are incorrect:

  • Option A: Expected but physically unstable configuration.


  • Option B: This is Manganese (\(Z=25\)).

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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