Final answer:
Ionization energy decreases down a group on the periodic table due to increased shielding of outer shell electrons and lower effective nuclear charge.
Explanation:
Ionization energy is the energy required to remove an electron from an atom in its gaseous state. The overall trend in ionization energy down a group on the periodic table is a decrease as you move from top to bottom due to the increased shielding of outer shell electrons and lower effective nuclear charge.
For example, Group 1 alkali metals exhibit lower ionization energies as you move down the group, making it easier to remove electrons compared to elements higher in the group.
Figuere 4.7.1 illustrates the periodic decrease in ionization energy within a group, emphasizing the trend seen in groups 1 and 18.
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