Some electron configurations deviate from the Aufbau principle due to the enhanced stability associated with half-filled and fully-filled sublevels.

The Aufbau principle is a key concept in chemistry that outlines the order in which electrons populate atomic orbitals within an atom. According to this principle, electrons fill the lowest energy levels first before progressing to higher energy levels. However, there are notable exceptions to this guideline, primarily driven by the stability conferred by half-filled and fully-filled sublevels.

A prominent example of this exception is chromium (Cr). According to the Aufbau principle, the expected electron configuration for chromium would be $[Ar] \, 4s^2 \, 3d^4$. However, the actual electron configuration is $[Ar] \, 4s^1 \, 3d^5$. The reason for this discrepancy is that half-filled sublevels (in this case, $3d^5$) exhibit greater stability than partially filled ones (such as $3d^4$). By transferring one electron from the $4s$ orbital to the $3d$ orbital, chromium attains a more stable electron configuration.

Another notable exception is copper (Cu). The Aufbau principle suggests that copper should have the electron configuration $[Ar] \, 4s^2 \, 3d^9$. However, the actual configuration is $[Ar] \, 4s^1 \, 3d^{10}$. Similar to chromium, copper achieves a more stable electron configuration by having a fully-filled $d$ sublevel ($3d^{10}$) instead of a partially filled one ($3d^9$). This stability is achieved by moving one electron from the $4s$ orbital to the $3d$ orbital.

These deviations arise primarily because the energy difference between the $4s$ and $3d$ orbitals is minimal. As a result, the stability gained from having half-filled or fully-filled sublevels outweighs the energy cost incurred when promoting an electron to a higher energy level. This phenomenon underscores the principle that nature tends to favor stability.

In summary, while the Aufbau principle provides a useful framework for predicting electron configurations, it does not fully account for the increased stability associated with half-filled and fully-filled sublevels. Consequently, certain elements, such as chromium and copper, exhibit electron configurations that diverge from those predicted by the Aufbau principle. Understanding these exceptions is essential for a deeper comprehension of atomic structure and electron configuration.