Ionization energy in the groups (from top to bottom) of the periodic table

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Ionization energy generally decreases as you move down a group because each step adds a new electron shell, making the atomic radius larger. The outermost electron sits farther from the nucleus and experiences more shielding from inner electrons, so the net pull of the nucleus on that electron is weaker. With less attraction binding the outer electron, less energy is needed to remove it. Across a period the opposite trend occurs, as increasing nuclear charge with similar shielding makes it harder to eject an electron. There can be small deviations due to electron configuration, but the overall pattern down a group is a decrease in ionization energy.