What is the definition of an anion?

An anion is specifically defined as an ion that carries a negative charge. This negative charge occurs because an anion has gained one or more electrons, resulting in an overall surplus of negative charge compared to the number of protons in the atom's nucleus. Anions are often formed from nonmetals, which tend to gain electrons to achieve a stable electron configuration, typically resembling that of the nearest noble gas. This is an essential concept in understanding ionic compounds, where anions bond with positively charged ions, known as cations, to form neutral compounds.

Other options present definitions that do not align with the established understanding of anions. For instance, an ion with no charge would be neutral and not classified as either an anion or a cation. An ion with a positive charge describes a cation, and ions do not exclusively form in solution, as they can exist in solid, liquid, or gaseous states under various conditions.