The addition of electron shells results in

shielding of electrons from the nucleus.

Addition of shells increases the distance of outer electrons from the nucleus.

Explanation:

Shielding effect is known as the attraction between the nucleus and an electron of any atom. In other words, it is the reduction in effective nuclear charge on an electron cloud.

Addition of electron shells results in the shielding of electron from nucleus. As the number of electron shells increases then farther will be the electrons placed from the nucleus and hence it will become easier to expel the electrons from outer shells with only little amount of ionization energy.

So, the amount of ionization energy require will be indirectly proportional to the shielding effect because more the shielding of electrons from the nucleus less will be the ionization energy require to expel the electrons.