An atom or group of atoms that bears an electric charge. Positively charged ions are called cations, and negatively charged ions are called anions. When a single atom gains or loses an electron, monoatomic ions are formed. For example, reaction of the element sodium (Na) with the element chlorine (Cl), which occurs as the diatomic gas Cl₂, leads to the transfer of electrons from Na to Cl to form Na⁺ cations and Cl⁻ anions, as shown in the reaction below.

Removal of an electron from an atom generates a species that has one more proton than electrons. For example, a sodium atom contains 11 negatively charged electrons and 11 positively charged protons. If one electron is removed, there will be 10 electrons and 11 protons, generating a positive charge on the sodium ion. Likewise, addition of an electron to an atom generates a species with more electrons than protons, which is therefore a negatively charged anion. In general, atoms of metallic elements (on the left side of the periodic table) lose electrons to form cations, while atoms of nonmetallic atoms (on the right side of the periodic table) gain electrons to form anions. Ions can bear multiple charges, as in the magnesium ion (Mg²⁺) or the nitride ion (N³⁻). The charge on monoatomic ions is usually the same for elements in the same column of the periodic table; for example, hydrogen (H), Na, lithium (Li), potassium (K), rubidium (Rb), and cesium (Cs) all form +1 ions. See also: Periodic table

Ions can also comprise more than one atom and are then called polyatomic ions. For example, the ammonium ion (NH₄⁺) carries a positive charge and is composed of one nitrogen atom and four hydrogen atoms. The nitrate ion (NO₃⁻) is composed of one nitrogen atom and three oxygen atoms and carries a single negative charge. Polyatomic ions are usually depicted inside brackets with superscripted charges, as shown in the structure below.

Anions and cations can combine to form solid materials called salts, which are named by the cation name followed by the anion name. For a salt composed of the polyatomic ions ammonium and nitrate, the formula is NH₄NO₃ and the name is ammonium nitrate. For monoatomic ions, the cation name is the same as the element and the anion name is the element name with the ending -ide. Thus, common table salt, NaCl, is called sodium chloride. The ratio of anions to cations must always be such that an electrically neutral material is produced. Thus, magnesium nitrate must contain one magnesium for every two nitrates, giving the formula Mg(NO₃)₂. See also: Salt (chemistry)

H. Holden Thorp