Answer:
Ionization energy is the energy required to remove the most loosely bound electron from an isolated gaseous atom.
The ionization energy decreases as the size increases. The electrons get added to the new shell and the nuclear charge increases. But the effect of addition of new sheels overcomes the increase of nuclear charge.The valence shell moves far from the nucleus.
Thus the valence electron gets less tightly held by the nucleus and thus less energy is required to remove the valence electron.
Shielding effect can be defined as a reduction in the effective nuclear charge on the electron cloud, due to a difference in the attraction forces of the electrons on the nucleus. Thus as the size increases and the electrons move away from nucleus, the attractive forces decrease.