Exceptional Electronic Configurations
Reasons | Half-Filled & Fully-Filled Stability | Class 11
1. Why Do Exceptions Occur?
The Aufbau principle correctly predicts the electronic configuration for most elements. However, in certain transition and inner-transition metals, an electron jumps from a lower energy $ns$ orbital to a higher energy $(n-1)d$ or $(n-2)f$ orbital. This happens because the energy gap between these orbitals is very small, and the jump results in a highly stable state.
There are two primary reasons for this extra stability:
A. Symmetrical Distribution of Electrons
Symmetry leads to stability. Completely filled or exactly half-filled subshells have a symmetrical distribution of electrons. This symmetry minimizes mutual shielding and maximizes nuclear attraction, lowering the overall energy of the atom.
B. Exchange Energy
Electrons in degenerate orbitals (orbitals with the same energy, like the five 3d orbitals) that have parallel spins can exchange their positions. Every exchange releases a small amount of energy called Exchange Energy.
(where $n$ is the number of electrons with parallel spins)
A $d^5$ configuration has 5 parallel electrons, resulting in $\frac{5(4)}{2} = 10$ exchanges. A $d^4$ configuration only has 6 exchanges. The higher exchange energy makes $d^5$ significantly more stable than $d^4$.
2. Exceptions in the 3d Series (Period 4)
These are the two most frequently asked exceptions in Class 11 and board exams.
| Element | Atomic No. ($Z$) | Expected Configuration | Actual Configuration | Reason |
|---|---|---|---|---|
| Chromium (Cr) | 24 | $[Ar] \ 4s^2 \ 3d^4$ | $[Ar] \ 4s^1 \ 3d^5$ | Extra stability of exactly half-filled $d^5$ subshell. |
| Copper (Cu) | 29 | $[Ar] \ 4s^2 \ 3d^9$ | $[Ar] \ 4s^1 \ 3d^{10}$ | Extra stability of completely filled $d^{10}$ subshell. |
3. Exceptions in Higher Series (JEE/NEET Specials)
As the principal quantum number increases, the energy gap between $s$, $d$, and $f$ orbitals becomes extremely small, leading to several anomalies.
Palladium ($Z=46$) is a unique element where two electrons shift from the $s$ orbital to the $d$ orbital.
Expected: $[Kr] \ 5s^2 \ 4d^8$
Actual: $[Kr] \ 5s^0 \ 4d^{10}$
Other Notable 4d & 5d Exceptions:
- Molybdenum (Mo, $Z=42$): Analogous to Cr. Actual = $[Kr] \ 5s^1 \ 4d^5$
- Silver (Ag, $Z=47$): Analogous to Cu. Actual = $[Kr] \ 5s^1 \ 4d^{10}$
- Gold (Au, $Z=79$): Analogous to Ag/Cu. Actual = $[Xe] \ 6s^1 \ 4f^{14} \ 5d^{10}$
The f-block Exception:
- Gadolinium (Gd, $Z=64$): Instead of putting the 8th electron into the $4f$ subshell to make $4f^8$, the electron enters the $5d$ subshell to maintain the highly stable half-filled $f^7$ core.
Expected: $[Xe] \ 6s^2 \ 4f^8$
Actual: $[Xe] \ 6s^2 \ 4f^7 \ 5d^1$
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