What is a negative ion called?

A negative ion is known as an anion. This term comes from the root "an-" which means "up," reflecting the behavior of such ions in a chemical reaction where they migrate towards the anode (the positive electrode) in an electric field. Anions are formed when an atom or molecule gains one or more electrons, resulting in a net negative charge due to the excess of electrons compared to protons. This gain of electrons typically occurs in non-metals, such as halogens, which tend to form stable anions by achieving a full outer electron shell.

The other terms do not correctly describe a negative ion. A proton refers specifically to a positively charged subatomic particle found in the nucleus of an atom. A cation is a positive ion, which has lost electrons rather than gained them. The phrase "negative charge" is a description of the charge of the anion itself, rather than a specific term for the ion. Therefore, an anion is the accurate term for a negatively charged ion.