Advanced Placement (AP) Chemistry Practice Exam

How does ionization energy change as you move down a group in the periodic table?

1. Ionization energy increases due to tighter electron binding

2. Ionization energy decreases due to increased electron shielding

3. Ionization energy remains constant across the group

4. Ionization energy decreases due to reduced atomic size

The correct answer is: Ionization energy decreases due to increased electron shielding

As you move down a group in the periodic table, ionization energy generally decreases due to increased electron shielding. The reason for this decrease is that as you add more electron shells with each subsequent element in the group, the outermost electrons are further from the nucleus. This increased distance reduces the effective nuclear charge felt by these outer electrons, making them easier to remove. Additionally, the inner shell electrons provide a shielding effect that further diminishes the attraction between the nucleus and the outermost electrons. As a result, it requires less energy to remove an electron from a larger atom than from a smaller one. Thus, the trend in ionization energy reflects the balance between the increased distance from the nucleus and the increased shielding from inner electrons, leading to a net decrease in ionization energy as you move down a group.