Final answer:
The ionization energy increases as the effective nuclear charge (Z-eff) increases because the electrons are held more tightly by the nucleus, requiring more energy to remove an electron.
Step-by-step explanation:
What happens to the ionization energy as the effective nuclear charge (Z-eff) increases? The correct answer is a) Increases. Ionization energy refers to the energy required to remove an electron from an atom in the gas phase. As Z-eff increases, the attractive force between the nucleus and the electrons strengthens, thereby increasing the energy required to remove an electron. This trend can be observed in the periodic table, where ionization energy tends to increase across a period due to the steady rise in Z-eff. This rise in Z-eff means that valence electrons are more strongly attracted to the nucleus, resulting in a decrease in atomic size and an increase in ionization energy.