Which characteristic best differentiates an anion from a cation?

An anion and a cation are differentiated primarily by their charge, which is directly related to the number of electrons they possess relative to their number of protons. An anion is formed when an atom gains one or more electrons, resulting in a negative charge. In contrast, a cation is formed when an atom loses one or more electrons, leading to a positive charge. Since the number of protons in an atom defines the element and cannot change without altering the element itself, it is the number of electrons that changes, leading to the distinction between anions and cations. Thus, the characteristic that best differentiates an anion from a cation is indeed the number of electrons.

The other options, while related to atomic structure, do not serve as a fundamental differentiator between these two types of ions. The number of protons and neutrons both relate to the identification of the element itself rather than its ionic state. Atomic radius can vary for both cations and anions but is not a definitive characteristic to distinguish between them. Therefore, the crucial factor lies in the differing numbers of electrons, making that the most precise answer.