Question

How Ionisation energy of an atom is affected by nuclear charge?

Answer 1

Ionization energy is the energy required to remove an electron from an isolated gaseous atom (X) in its ground state.

Across the period, the increasing nuclear charge outweighs the shielding. Consequently, the outermost electrons are held more and more tightly by the nucleus and the ionization energy increased across the period.

On moving down the group, the outermost $e^{-}$ begin increasingly farther from the nucleus, there is an increased shielding of the nuclear charge by the electrons in the inner levels. In this case, increase in shielding outweighs the increasing nuclear charge and the removal of thee outermost $e^{-}$ requires less energy. Hence, ionization energy decreases down the group.