Which type of ion is associated with the smallest ionization energy?

The correct answer is an anion. Ionization energy refers to the energy required to remove an electron from an atom or ion in its gaseous state. When we consider anions, which are negatively charged species formed when atoms gain electrons, the effect of electron-electron repulsion within the ion is greater due to the additional electron(s). This makes it easier to remove an electron compared to cations or neutral atoms, which typically have fewer electrons and, in the case of cations, have a net positive charge due to the loss of electrons.

Neutral atoms have a higher ionization energy compared to anions because they have a balanced number of protons and electrons, leading to a more stable electron configuration. Cations, formed by losing electrons, generally hold their remaining electrons more tightly due to the increased effective nuclear charge perceived by the remaining electrons, resulting in higher ionization energies.

In summary, an anion's additional electron(s) lead to stronger repulsive forces that lower the energy required to remove an electron compared to cations and neutral atoms, making it associated with the smallest ionization energy.