Question

for element x represented above, which of the following is the most likely explanation for the large difference between the second and third ionization energies?
a the effective nuclear charge decreases with successive ionizations.
b the shielding of outer electrons increases with successive ionizations.
c the electron removed during the third ionization is, on average, much closer to the nucleus than the first two electrons removed were.
d the ionic radius increases with successive ionizations.

Explanation:

Ionization energy is the energy required to remove an electron from an atom or ion. The large difference between the second and third ionization energies indicates a significant change in the electron - removal process. When electrons are removed, the remaining electrons experience less shielding. However, after removing two electrons (to form \(X^{2+}\)), removing the third electron (to form \(X^{3+}\)) is much more difficult. This is because the electron removed during the third ionization is, on average, much closer to the nucleus. As electrons are removed, the positive charge of the nucleus (protons) remains the same, but the number of electrons decreases. So, the effective nuclear charge experienced by the remaining electrons increases. But the key factor for the large jump in ionization energy (from \(IE_2\) to \(IE_3\)) is that the electron being removed in the third ionization is in a shell closer to the nucleus (e.g., if the first two electrons were in a higher - energy shell, and the third is in a lower - energy shell closer to the nucleus).

• Option A: The effective nuclear charge increases with successive ionizations as electrons are removed and the shielding decreases. So, this option is incorrect.
• Option B: Shielding of outer electrons decreases with successive ionizations (as there are fewer electrons to shield), so this option is incorrect.
• Option D: Ionic radius decreases with successive ionizations (as more positive charge pulls the remaining electrons closer), so this option is incorrect.

Answer:

C. The electron removed during the third ionization is, on average, much closer to the nucleus than the first two electrons removed were.