Why does boron have a lower ionization energy than beryllium?

• A. The filled s subshell of beryllium is more stable.
• B. Electrons in the subshells of beryllium experience more repulsions.
• C. Beryllium has a smaller effective nuclear charge (Zeff) than boron.
• D. Boron has a smaller atomic radius which makes it easier to remove a valence electron.

Answer: (A)