Although the 4s orbitals are occupied before 3d orbitals, yet we cannot say that they
are always more stable. Explain with suitable examples.
The 4s sublevel is only lower in energy if there are no electrons in the 3d sublevel. According to the Aufbau principle, the 4s sublevel is filled before the 3d sublevel because the 4s is lower in energy. As the 3d sublevel becomes populated with electrons, the relative energies of the 4s and 3d fluctuate relative to one another and the 4s ends up higher in energy as the 3d sublevel fills. This is why when electrons are lost from