The filling order of electron shells is determined by the Aufbau principle, which states that electrons occupy the lowest available energy level before filling higher energy levels. However, there are some exceptions to this principle due to the specific orbital structures and electron-electron interactions.
In the case you mentioned, where the 4th shell is filled before the 3rd shell, it is likely due to the electron configuration of the elements involved. The 4th shell corresponds to the n = 4 energy level, while the 3rd shell corresponds to the n = 3 energy level.
One common example of this exception is observed in transition metals. Transition metals have a partially filled d subshell, which is located in the 3rd shell (n = 3). As a result, when filling the electron shells, the d subshell is typically filled after the 4s subshell in transition metals.
The reason for this is related to the relative energy levels of the orbitals. In certain cases, the energy of the 4s orbital is lower than the energy of the 3d orbitals, allowing electrons to occupy the 4s subshell before the 3d subshell. This is known as the "energy-level inversion" or "energy-level crossover" phenomenon.
It's important to note that this exception is not a general rule and is specific to transition metals and their electron configurations. In most other cases, electrons follow the Aufbau principle, filling the lower energy levels first before moving to higher energy levels.