How is a negative ion formed?

A negative ion, also known as an anion, is formed specifically when an atom or molecule gains one or more electrons. Electrons carry a negative charge, so adding them increases the overall negative charge of the atom or molecule, resulting in the formation of a negatively charged ion.

In contrast, losing a proton does not affect the overall charge in the same way since protons reside in the nucleus and are positive. When an atom loses an electron, it becomes positively charged, forming a cation rather than an anion. Sharing electrons typically occurs in covalent bonding, where atoms form molecules by overlapping their electron clouds rather than gaining or losing electrons completely, which would not lead to the formation of an ion. Therefore, gaining electrons is the defining process for the creation of a negative ion.