In the case of many electrons, the electrons in the atom can protect the electrons of the valence shell from the pull of the nucleus. This effect is known as the shielding effect (also known as the screening effect).

Electron Shielding Effect

The shielding effect is just like a protective coat which results in the decrease in the nuclear charge felt by the valence electrons.

Shielding effect or Screen effect affects the ionization energies

An increase in the shielding effect requires less ionization energy to remove the electron from an isolated neutral gaseous atom. This is because the ionization energy is the energy required to remove the electron from an isolated neutral gaseous atom, thus ionizing the atom.

Atoms with less shielding have their own electrons closer to the nucleus and therefore require more energy to remove the outer electron e.g. helium and atoms such as Cesium and Francium have a low pull or force of attraction between the valence shell electron and the nucleus, so they require less ionization energy to remove the outer electron.

Conclusion:

More inner electrons shield the effect of the nucleus on the valence electrons, resulting in a bigger atomic size and need less ionization energy.
Less inner electrons increase the pull of the nucleus on the valence electrons, resulting in a smaller atomic size and the need for more ionization energy.