Why are there exceptions in electron configurations for certain elements?

Exceptions in electron configurations occur due to the stability achieved by half-filled or fully-filled sublevels.

In more detail, electron configurations of atoms are typically predicted by the Aufbau principle, which states that electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels. However, there are certain elements that do not follow this principle and have exceptions in their electron configurations. This is primarily due to the stability that is achieved by having half-filled or fully-filled sublevels.

For instance, chromium and copper are two elements that have exceptional electron configurations. Chromium, with atomic number 24, is expected to have the electron configuration [Ar] 4s2 3d4 according to the Aufbau principle. However, it is actually [Ar] 4s1 3d5. This is because by moving one electron from the 4s orbital to the 3d orbital, chromium achieves a half-filled 3d sublevel, which provides extra stability.

Similarly, copper, with atomic number 29, is expected to have the electron configuration [Ar] 4s2 3d9. But its actual configuration is [Ar] 4s1 3d10. By moving one electron from the 4s orbital to the 3d orbital, copper achieves a fully-filled 3d sublevel, which is more stable.

The concept of half-filled and fully-filled sublevels being more stable is related to the exchange energy, a concept in quantum mechanics. The exchange energy is the energy associated with the swapping of identical particles. In the case of electrons in an atom, the exchange energy is lower (and thus more stable) when the electrons are in different orbitals with parallel spins. Therefore, half-filled and fully-filled sublevels, where each orbital within the sublevel has one electron (half-filled) or two electrons with opposite spins (fully-filled), are more stable due to the lower exchange energy.

IB Chemistry Tutor Summary: In summary, some elements have unusual electron configurations because half-filled or fully-filled sublevels offer extra stability. Despite the Aufbau principle suggesting a certain order for filling electrons, elements like chromium and copper adjust their electron configurations to achieve this stability. This special arrangement is due to exchange energy, making these configurations more energetically favourable.