Nuclear charge actually experienced by an electron is termed as effective nuclear charge…

Nuclear charge actually experienced by an electron is termed as effective nuclear charge. The effective nuclear charge Z* actually depends on type of shell and orbital in which electron is actually present. The relative extent to which the various orbitals penetrate the electron clouds of other orbitals is.
s > p > d > f (for the same value of n)
The phenomenon in which penultimate shell electrons act as screen or shield in between nucleus and valence shell electrons and thereby reducing nuclear charge is known as shielding effect. The penultimate shell electrons repel the valence shell electron to keep them loosely held with nucleus. It is thus evident that more is the shielding effect, lesser is the effective nuclear charge and lesser is the ionization energy.