American Chemical Society (ACS) Chemistry Practice Exam

An ion is distinguished from a neutral atom by the change in the number of electrons. Neutral atoms have an equal number of protons and electrons, which balances the positive and negative charges. When an atom gains or loses electrons, it becomes charged and is thus classified as an ion. If an atom gains one or more electrons, it becomes a negatively charged ion (anion), while losing electrons results in a positively charged ion (cation).

This fundamental difference in electron count, while the number of protons remains unchanged, is critical because it directly affects the atom's overall charge and chemical reactivity. The mass of the ion remains similar to that of the neutral atom because the mass of electrons is negligible compared to that of protons and neutrons. Additionally, the structure of the atom, referring to its arrangement of electrons around the nucleus, can also vary with different ionization states, but the defining characteristic is the number of electrons relative to protons. Changes in protons would alter the identity of the element altogether, which is not the case when forming ions.