What defines an ion?

An ion is specifically defined as an atom that has gained or lost electrons, resulting in a net electrical charge. When an atom loses electrons, it becomes positively charged and is referred to as a cation, while gaining electrons makes it negatively charged, known as an anion. This alteration in the balance between protons and electrons dictates the charge of the atom, distinguishing ions from neutral atoms, which have an equal number of protons and electrons and thus no overall charge.

The other options do not accurately define an ion. An atom with no charge is simply neutral, while a stable and unreactive atom refers to an atom in a stable electronic configuration, typically a noble gas, which does not necessarily involve gaining or losing electrons. Similarly, an atom with a stable nucleus relates to the atomic structure but does not pertain to the specific properties that define ions.