Why are heavier noble gases easier to ionize than lighter noble gases?

Heavier noble gases are easier to ionize than lighter ones primarily due to the increase in atomic radius as you move down the group in the periodic table.1

Ionization energy is the minimum energy required to remove an electron from an atom. This process is influenced by the distance between the valence electrons and the nucleus:12

• Atomic Size: In lighter noble gases like helium and neon, valence electrons are located closer to the nucleus, where they experience a strong attractive force, resulting in very high ionization energies.1
• Distance and Shielding: As atoms become larger (heavier), the valence electrons are, on average, much farther from the nucleus. This increased distance weakens the electrostatic attraction between the nucleus and the electrons, making it easier to remove an electron.1

Consequently, while the lighter members of the family have some of the highest ionization energies of all elements, the ionization energies of heavier noble gases like krypton and xenon are significantly lower, allowing them to participate in chemical compound formation under specific conditions.1