before discuss on the ionization of noble gas, we need to define and explain what ionization is all about.
The ionization energy (Ei) can defined primarily as the highest amount of energy needed to remove the most loosely bound electron in the valence electron, of an isolated neutral gaseous atom to yield cation. according to Wikipedia It is quantitatively expressed in symbols as
X + energy → X+ + e−
where X is any atom or molecule capable of being ionized, X+ is that atom or molecule with an electron removed, and e− is the removed electron. This is an endothermic process.
Generally, the closer the electrons are to the nucleus of the atom, the higher the atom's ionization energy.
with above definition, we now understand ionization energy.
noble gas ionization energy
due to the configuration of noble gas, noble tend to have high ionization energy compare to other element in the period in the periodic table, this what contribute to there reluctant chemical reactivity. but on going down the group of the noble gas, there is decrease in the ionization energy, as such lead to increase in there chemical reactivity.