Concept:Ionization energy is the energy needed to remove the outermost electron from a gaseous atom.
Explanation:Across a group, ionization energy decreases as atomic size increases.
Down the noble gases group, the atomic radius increases.
Added electron shells increase the shielding effect, weakening the nuclear attraction on outer electrons.
Thus, outer electrons become easier to remove as we move downward.
Helium is the smallest noble gas with only one electron shell and strong nuclear pull.
Therefore, helium has the highest ionization energy in the group.
Answer:Helium (
He) has the greatest ionization energy.