Why are some electron configurations exceptions to the Aufbau principle?

Some electron configurations are exceptions to the Aufbau principle due to the stability of half-filled and fully-filled sublevels.

The Aufbau principle is a fundamental concept in chemistry that describes how electrons fill up atomic orbitals in an atom. According to this principle, electrons occupy the lowest energy levels first, before moving on to higher energy levels. However, there are certain exceptions to this rule, primarily due to the stability that half-filled and fully-filled sublevels offer.

The first exception is chromium (Cr). According to the Aufbau principle, the electron configuration of chromium should be [Ar] 4s2 3d4. However, the actual electron configuration is [Ar] 4s1 3d5. This is because half-filled sublevels (d5 in this case) are more stable than partially filled ones (d4). By moving one electron from the 4s orbital to the 3d orbital, chromium achieves a more stable electron configuration.

Another exception is copper (Cu). The Aufbau principle predicts the electron configuration of copper to be [Ar] 4s2 3d9. However, the actual electron configuration is [Ar] 4s1 3d10. Similar to chromium, copper achieves a more stable electron configuration by having a fully-filled d sublevel (d10) instead of a partially filled one (d9). This is achieved by moving one electron from the 4s orbital to the 3d orbital.

These exceptions occur because the energy difference between the 4s and 3d orbitals is very small. Therefore, the stability gained by having half-filled or fully-filled sublevels outweighs the energy required to promote an electron to a higher energy level. This is a testament to the fact that nature favours stability. For further understanding of how electron shells are structured, you can read about the s, p, d, and f blocks.

IB Chemistry Tutor Summary: The Aufbau principle usually explains how electrons fill low energy levels first. Yet, exceptions like chromium and copper exist because half-filled or fully-filled sublevels are more stable. These atoms adjust their electron configurations to achieve stability, despite the Aufbau principle's predictions. This shows nature's preference for stable arrangements, even if it means adjusting expected electron placements. For a detailed explanation of the Aufbau principle and its exceptions, refer to our notes on the Aufbau principle and exceptions to electron configurations.