Ions and Ionization, An ion is an electrically charged atom or group of atoms. A normal atom is electrically neutral, because it has the same number of electrons (particles bearing a negative charge) as protons (particles bearing a positive charge). The atom becomes a negative ion, or anion, upon gaining one or more electrons. The atom becomes a positive ion, or cation, when it loses one or more electrons. A molecule or a radical (two or more atoms that act as a unit in forming a molecule) may also become an ion.

Ionization is the process by which an electrically neutral atom, molecule, or radical loses or gains one or more electrons and becomes an ion. Ionization can occur in gases, liquids, or solids. Physicists often call a highly ionized gas a plasma.

Metals tend to form positive ions, nonmetals negative ions. An amphoteric ion carries both a positive and a negative charge, usually at opposite ends of a long chain structure. Most amphoteric ions are organic (contain carbon). Ions of amino acids are typical amphoteric ions.