Concept:Ionization energy is the minimum energy required to remove an electron from a gaseous atom in its ground state, forming a cation.
Explanation:Across a period, nuclear charge increases and atomic radius decreases, so ionization energy generally increases.
Down a group, atomic radius increases and electron shielding increases, so the outer electron is held less tightly and ionization energy decreases.
Removing an electron produces a positively charged cation, not an anion.
Ionization energy does not cause metallic nuclei to disintegrate.
Therefore, the only correct statement is that ionization energy decreases down the group.
Answer:D. decreases down the group