Which of the following best explains why the ionization energy decreases as you move down a group in the periodic table?

• A. The shielding effect decreases, making it easier to remove an electron.
• B. The number of protons decreases, resulting in a weaker attraction between the nucleus and electrons.
• C. The atomic radius increases, causing the outer electrons to be farther from the nucleus and less tightly held.
• D. The atoms gain more electrons in the outer shell, increasing the ionization energy.

Answer: (C)

Verified step by step guidance

1. Understand that ionization energy is the energy required to remove an electron from an atom in its gaseous state.
2. Recognize that as you move down a group in the periodic table, atoms have more electron shells, which increases the atomic radius.
3. Know that an increased atomic radius means the outermost electrons are farther from the nucleus, reducing the electrostatic attraction between the nucleus and these electrons.
4. Consider the shielding effect, where inner electrons partially block the attraction between the nucleus and outer electrons, which also increases down a group and lowers ionization energy.
5. Conclude that the combination of increased atomic radius and increased shielding effect makes it easier to remove an electron, thus decreasing the ionization energy as you move down a group.