Why do electrons enter the 4s orbital before the 3d orbital?

Electrons enter the 4s orbital before the 3d orbital because the 4s orbital has slightly lower energy than the 3d orbital in neutral atoms. According to the Aufbau principle, electrons fill orbitals in order of increasing energy to achieve a more stable arrangement.

Although the 3d orbital belongs to a lower principal energy level, its energy is higher than the 4s orbital before filling begins. Therefore, after the 3p orbital is filled, electrons occupy the 4s orbital first, followed by the 3d orbital.

This filling order is observed in elements such as potassium and calcium, where electrons enter 4s before transition metals begin filling the 3d subshell. However, after electrons occupy the 3d orbital, the relative energy order changes, and 4s electrons are usually removed first during ion formation.

This concept helps explain electronic configurations and the properties of transition elements.