What defines an anion?

An anion is defined specifically as an ion that carries a negative charge. This occurs when an atom or a molecule gains one or more electrons. Electrons are negatively charged particles, so when they are added to a neutral atom, the overall charge of that atom becomes negative, resulting in the formation of an anion.

For example, when a chlorine atom (which normally has 17 electrons) gains an extra electron, it becomes a chloride ion (Cl−) with a single negative charge. The process of gaining electrons is key to understanding the nature of anions, as it differentiates them from cations, which are positively charged ions resulting from the loss of electrons.

In the context of the other options, an ion with a positive charge describes cations, while an atom with no charge refers to neutral atoms, and a particle that does not participate in chemical reactions does not accurately reflect the behavior of anions, which can engage in various chemical interactions depending on the conditions. Thus, the statement about an anion specifically highlights its formation through gaining electrons and its resulting negative charge.