What defines an ion?

An ion is specifically defined as an atom that has a charge due to the loss or gain of one or more electrons. When an atom loses electrons, it becomes positively charged, known as a cation. Conversely, when an atom gains electrons, it becomes negatively charged and is referred to as an anion. This change in the number of electrons relative to protons causes the atom to no longer be electrically neutral, resulting in the formation of an ion. Thus, the core characteristic of an ion is its alteration in electron count, which leads to the charge.

The other options refer to different concepts: an atom with a positive charge refers specifically to cations, while an atom with equal numbers of protons and electrons remains neutral and is not an ion. The notion of atoms always being found in pairs does not pertain to the definition of an ion at all, as ions can exist independently in a solution or in gaseous form. Therefore, the defining aspect of an ion is the process of losing or gaining electrons, leading to a charge.