Which element in the second period has the highest first ionization energy?

• A. Lithium
• B. Beryllium
• C. Boron
• D. Neon

Answer: (D)

Ionization energy is the energy needed to remove the outermost electron, and across a period the nuclear charge increases while shielding stays relatively constant, so the remaining electrons are pulled in more tightly. In the second period, this effect makes Neon the hardest to remove an electron because it has a complete valence shell (an octet), giving it exceptional stability. Taking away one electron from Neon would disrupt that closed shell, requiring a lot more energy than removing an electron from lithium, beryllium, or boron, whose outer electrons are less tightly bound. That combination of a full, stable shell and a high effective nuclear charge explains why Neon has the highest first ionization energy among these elements.