The particle that an atom can lose or gain in order to form a charged ion is an electron. When an atom loses electrons, it becomes a positively charged ion called a cation because the number of protons exceeds the number of electrons. Conversely, when an atom gains electrons, it becomes a negatively charged ion called an anion because the number of electrons exceeds the number of protons. This change in electron number causes the atom to have an overall electric charge, forming an ion.