What is an ion?

An ion is defined as an atom that has gained or lost electrons, resulting in a net electrical charge. When an atom loses one or more electrons, it becomes a positively charged ion, known as a cation. Conversely, if an atom gains electrons, it becomes a negatively charged ion, known as an anion. This ability to lose or gain electrons is fundamental to the formation of ionic bonds and the behavior of atoms in chemical reactions.

In contrast, the other options refer to different concepts. A molecule missing a neutron does not define an ion; such a molecule would be better described in terms of isotopes. A negatively charged particle in an atom typically refers to an electron itself, not an ion, since a complete ion can be either negatively charged (anion) or positively charged (cation). An atom with equal numbers of protons and electrons is neutral and, therefore, not classified as an ion. Thus, recognizing that an ion is specifically about the loss or gain of electrons clarifies the concept and solidifies the understanding of ion formation.