Why does ionization energy decrease as you move down a group in the periodic table?

• A. Because the number of valence electrons decreases down a group.
• B. Because the outer electrons are farther from the nucleus and experience greater shielding from inner electrons.
• C. Because the nuclear charge decreases as you move down a group.
• D. Because atoms gain more protons, 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, meaning the outermost electrons are farther from the nucleus.
3. Know that inner electrons shield the outer electrons from the full positive charge of the nucleus, reducing the effective nuclear charge felt by the valence electrons.
4. Realize that because the outer electrons are farther away and more shielded, they are held less tightly, so less energy is needed to remove them.
5. Conclude that the decrease in ionization energy down a group is due to increased distance and shielding, not a decrease in the number of valence electrons or nuclear charge.