Atoms, or groups of atoms, which have either taken on or given up one or more orbital electrons. As a result of the gain or loss of one or more electrons, each ion bears electrical charge. Positively charged ions are called cations and negatively charged ions are called anions.

In solutions, the ions interact with the solvent to become hydrated and the properties of the ion become modified by the solvent molecules attached in varying degrees to the ion. Other than in solutions, ions may exist in the solid state as in a crystal lattice; or in the [[gas|gaseous]] state, where they are produced by the action of high-energy [[radiation]] on neutral atoms or molecules of the gas.