What defines an ion?

An ion is defined specifically as an electrically charged particle that results from the loss or gain of electrons by an atom. When an atom loses one or more electrons, it becomes positively charged, forming a cation. Conversely, when an atom gains one or more electrons, it develops a negative charge and is referred to as an anion. The defining characteristic of an ion is this imbalance between the number of protons (positively charged) and electrons (negatively charged), leading to an overall charge.

In relation to the other definitions, while an unpaired electron (as mentioned in one of the choices) might indicate a radical or an atom that is highly reactive, it does not directly correlate to the concept of an ion. The notion of a full outer shell of electrons pertains more to the stability of atoms, but again, does not characterize ions. Lastly, an atom with an equal number of protons and neutrons focuses on the atomic structure rather than charge, and it is possible for such an atom to still be neutral without being an ion. Thus, the correct definition is that an ion is formed through the charge resulting from the gain or loss of electrons.