Electrons in outer energy levels can still penetrate the shielding of inner energy levels if the orbitals have a radial distribution close to the nucleus.
- For example, 3s can penetrate the shielding better than 3p since the former has a higher radial distribution closer to the nucleus.
- 4s is filled first before 3d because 4s penetrates the shielding better (4s has slightly higher radial distribution for very small r) even though the 3d’s most probable distance is closer to the nucleus than 4s.