Which ionization energy requires the most energy to remove an electron from an atom?

• A. First ionization energy
• B. Fourth ionization energy
• C. Second ionization energy
• D. Third ionization energy

Answer: (D)

Verified step by step guidance

1. Understand that ionization energy is the energy required to remove an electron from an atom or ion in the gas phase.
2. Recognize that the first ionization energy refers to removing the first electron from a neutral atom, the second ionization energy is for removing a second electron after the first has been removed, and so on.
3. Know that as electrons are removed, the remaining electrons experience a stronger effective nuclear charge because there are fewer electrons to shield the nucleus, generally causing ionization energies to increase with each successive electron removed.
4. Identify that the ionization energy that requires the most energy is usually the one where an electron is removed from a more stable, often a closed-shell or noble gas configuration, which is harder to disrupt.
5. Therefore, the third ionization energy is higher than the first, second, and fourth in this context because removing the third electron likely involves breaking into a more stable electron configuration, requiring significantly more energy.