What is an anion?

An anion is defined as a negatively charged ion that has gained electrons. This occurs because when an atom or molecule acquires additional electrons, it results in an overall negative charge due to the imbalance between the number of protons (positively charged) and electrons (negatively charged).

In the process of gaining electrons, the atom or molecule may achieve a more stable electron configuration, often resembling that of a noble gas. For instance, a chlorine atom can gain one electron to become a chloride ion, resulting in a negatively charged anion. This fundamental concept illustrates why the identification of anions is crucial in understanding ionic compounds and their properties in chemistry.

The other options describe different concepts that do not pertain to the definition of anions.