How does shielding effect affect ionization energy?

Hint:As we know ionization energy is the quantity of energy that an isolated, gaseous atom in the ground electronic state must absorb to discharge an electron, which results in formation of a cation. This energy is expressed in $KJ/mol$, or the amount of energy it takes for all the atoms in a mole to lose one electron each.

Complete answer:
Alkali metals have small ionization energies when they are compared to halogens. In addition to the radius which is as we know the distance between the nucleus and the electrons in the outermost orbital, the number of electrons between the nucleus and the electrons you are looking at in the outermost shell have an effect on the ionization energy as well.
This effect, where the full positive charge of the nucleus is not experienced by outer electrons due to the negative charges of the inner electrons partially canceling out the positive charge is called shielding.
The more electrons shielding the outer electron shell from the nucleus lesser is the energy required to expel an electron from the atom. The higher the shielding effect the lower the ionization energy. Shielding or the screening effect basically acts as a barrier effect.
This is because of the shielding effect that the ionization energy decreases from top to bottom within a group. Hence, we can say that cesium is said to have the lowest ionization energy and fluorine is said to have the highest ionization energy.

Note:There are three factors which affect the ionization energy.
 Size of the positive nuclear charge - As the nuclear charge increases, ionization energy increases.
 Size of an atom - As the atomic size increases, the ionization energy decreases.
Shielding effect of inner shell electrons - As shielding increases, the ionization energy decreases.