Answer:
The phenomenon to which you are alluding in the context of electron configuration and atomic structure is known as the "anomalous electron configuration" or "exception to the Aufbau principle." This happens when an electron from a level with lower energy fills an orbital in a level with higher energy.
The electron arrangements of copper (Cu) and chromium (Cr) are the most prevalent examples of this:
Chromium (Cr): In accordance with the Aufbau principle, chromium should have an electron configuration of [Ar] 4s2 3d4. However, it has been shown that chromium's real electron configuration is [Ar] 4s1 3d5. Because a partially filled 3d orbital, which has lower energy, is more stable than a fully filled 4s orbital, this is the case.
Similar to copper, the electron configuration of copper should be [Ar] 4s2 3d9, but it is actually [Ar] 4s1 3d10. Similar to chromium, copper has a fully-filled third-order orbital in its arrangement, which increases stability.
In both situations, a partially filled or fully filled d orbital has a lower energy than the customary pattern of filling the s orbital first and then going to the d orbitals.
This phenomenon emphasizes the significance of electron-electron interactions and the need to organize electrons in orbitals to increase stability, even if doing so requires departing from the straightforward Aufbau principle.