Which best explains why ionization energy tends to decrease from the top to the bottom of a group in the periodic table?

• A. The number of valence electrons increases, making ionization easier.
• B. Atomic radius increases, so the outer electrons are farther from the nucleus and less tightly held.
• C. Electron shielding decreases, causing electrons to be more easily removed.
• D. Nuclear charge decreases, making it easier to remove an electron.

Answer: (B)

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 a larger atomic radius means the outermost electrons are farther from the nucleus, reducing the electrostatic attraction between the nucleus and these electrons.
4. Consider the effect of electron shielding, where inner electrons repel outer electrons, further decreasing the effective nuclear charge felt by the outermost electrons.
5. Conclude that because the outer electrons are less tightly held due to increased distance and shielding, ionization energy decreases from top to bottom within a group.