Anomalous Configuration

Question

Which element shows anomalous electron configuration due to stability of half-filled d-subshell?

RankGuru · Accepted Answer

1. **Aufbau Expectation**: Chromium ($Z=24$) would be expected to be $[Ar] 3d^4 4s^2$.
2. **Experimentation**: The actual configuration is **$[Ar] 3d^5 4s^1$**.
3. **Reason**: 
   - **Symmetry**: A half-filled $d$-subshell ($d^5$) is symmetrical.
   - **Exchange Energy**: Electrons with parallel spins can 'exchange' positions, which is a stabilizing process. $d^5$ allows more exchange pairs than $d^4$.
4. **Outcome**: The stability gained by filling the $3d$ subshell half-way exceeds the energy required to promote an electron from $4s$.

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Anomalous Configuration

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