Which area of the periodic table shows greater ionization energy?

• A. Right side and up
• B. Left side and down
• C. Left and up
• D. Right and down

Answer: (A)

Ionization energy is the energy needed to remove an electron from a gaseous atom. As you move across a period from left to right, the number of protons increases, pulling valence electrons closer and increasing the effective nuclear charge felt by them, while shielding doesn’t rise much. This makes it harder to remove an electron, so ionization energy goes up. Moving down a group adds electron shells, increasing shielding and distance from the nucleus, which makes it easier to remove an electron, so ionization energy decreases. Put together, the area with the greatest ionization energy is the top-right region of the periodic table, where atoms are small and electrons are held most tightly. Helium, at the very top, exemplifies the peak in this trend.