What is the Orbital Penetration Effect?
The extent to which an orbital of a shell interacts with the lower quantum shell orbitals is referred to as the orbital penetration.
We know that the energy of the 4s orbital is lower than that of 3d orbital and that the filling of 4s orbital begins even before the third shell has been completely filled, This is known as the penetration of the 3d orbitals by the 4s orbital. The penetration is more as the number of shell increases. Thus 5s orbital penetrates the 4d and 4f orbitals. The penetration of the orbitals of a given quantum shell follows the following order: s > p> d > f