* Ionization energy is the energy required to remove an electron from an atom in its gaseous state.

* Beryllium (Be) is in the second period of the periodic table, while Barium (Ba) is in the sixth period.

* Atomic size: Barium is significantly larger than beryllium. Electrons in barium's outermost shell are farther from the nucleus and experience weaker electrostatic attraction.

* Shielding effect: The inner electrons in barium shield the outermost electrons from the positive nucleus more effectively than beryllium's inner electrons.

Therefore, it takes less energy to remove an electron from barium, making its ionization energy lower than beryllium's.