Why is the second ionization energy greater than the first ionization energy?

A. The size of atoms increases down a group.

B. The size of anions decreases across a period.

C. It is more difficult to remove a second electron from an atom.

D. The nuclear attraction from protons in the nucleus decreases.

C. It is more difficult to remove a second electron from an atom.

Your answer is C.

because The reason the second ionization energy is higher than the first relates to the attraction between the electrons and the nucleus. When one electron is removed from an atom, the neutral atom becomes positive ... Thus, the second electron is harder to remove and the ionization requires more energy.