Which of the following is an example of an anion?

An anion is defined as a negatively charged ion. The important characteristic of an anion is that it has gained one or more electrons, which results in a net negative charge. In this case, the chloride ion (Cl-) has a single negative charge, indicating it has gained an electron compared to its neutral state. Hence, it qualifies as an anion.

On the other hand, the other options represent cations, which are positively charged ions. For instance, both sodium (Na+), calcium (Ca2+), and magnesium (Mg2+) ions have lost electrons, resulting in their respective positive charges. This distinction between cations and anions is crucial for understanding their roles in chemical reactions, particularly in ionic bonding and the behavior of compounds in solution.