Chromium and copper are exceptions to the simplest filling prediction because the 3d and 4s sublevels are extremely close in energy. Moving one 4s electron into 3d produces a lower-energy, more stable arrangement.
The Reasoning
The Aufbau principle predicts that electrons occupy available orbitals in order of increasing energy. Applying the basic filling order gives configurations that differ from the experimentally observed configurations:
| Element | Simple prediction | Observed configuration |
|---|---|---|
| Chromium, Cr | [Ar] 3d⁴ 4s² | [Ar] 3d⁵ 4s¹ |
| Copper, Cu | [Ar] 3d⁹ 4s² | [Ar] 3d¹⁰ 4s¹ |
For chromium, transferring one electron from 4s to 3d gives five singly occupied 3d orbitals. This half-filled d sublevel has relatively low electron-electron repulsion and additional stability associated with the distribution of electrons among equivalent orbitals.
For copper, the transfer produces five completely occupied 3d orbitals. This fully filled d sublevel is lower in energy than the predicted 3d⁹ 4s² arrangement.
The key point is that the Aufbau filling order is a model, not an unbreakable rule. Because the 3d and 4s energies are very similar, small differences in electron-electron repulsion and orbital stability can change which configuration has the lowest total energy.
A common misconception is that chromium and copper “break” the Aufbau principle. They do not: electrons still adopt the lowest-energy arrangement, but the simplified filling diagram does not perfectly represent the energies in multi-electron atoms.
Exam Technique
This Structure 1.3 explanation supports both SL and HL understanding. If asked to explain, state the predicted and observed configurations, identify the similar energies of 3d and 4s, and connect the change to the stability of half-filled or fully filled 3d sublevels. Also remember that when transition-metal ions form, 4s electrons are removed before 3d electrons.