anion (ănˈĪˌən) [key], atom or group of atoms carrying a negative charge. The charge results because there are more electrons than protons in the anion. Anions can be formed from nonmetals by reduction (see oxidation and reduction) or from neutral acids (see acids and bases) or polar compounds by ionization. Anionic species include Cl - , SO4 - - , and CH3COO - . Highly colored intermediates in organic reactions are often radical anions (anions containing an unpaired electron). Salts are made up of anions and cations. See ion.