an atom or goup of atoms (radical) carrying an electrical charge. It is contrasted with neutral atoms or molecules, and free radicals. Certain compounds, such as sodium chloride, are composed of complementary ions in the solid (crystalline) as well as in solution. Others, notably acids such as hydrogen chloride, may occur as neutral molecules in the pure liquid or gas forms, and ionize almost completely in dilute aqueous solutions. In solutions (as in water) ions are frequently bound non-covalently with the molecules of solvent, and in that case are said to be solvated. According to the electrolytic dissociation theory, the molecules of electrolytes are divided into ions by water and other solvents. An ion consists of one or more atoms and carries one unit charges of electricity, 3.4 x 10-10 electrostatic units, or a multiple of this. Those which are positively electrified (hydrogen and the metals) are called cations; negative ions (hydroxyl and acidic atoms or groups) are called anions.

One of the small electrified particles into which the molecules of a gas are broken up under the action of the electric current, of ultraviolet and certain other rays, and of high temperatures. To the properties and behavior of ions the phenomena of the electric discharge through rarefied gases and many other important effects are ascribed. At low pressures the negative ions appear to be electrons; the positive ions, atoms minus an electron. At ordinary pressures each ion seems to include also a number of attached molecules. Ions may be formed in a gas in various ways.

Ions are atoms which have lost or gained electrons in their outer electron shell. If an atom accepts electrons anions are formed, if an atom loses electrons cations are formed. See also explanations on ionization and electronegativity

An electrically charged atom or group of atoms due to the loss or gain of one or more electrons. Positively charged ions, known as cations, include potassium, calcium, sodium, hydrogen, ammonium, and magnesium. Negatively charged ions, called anions, include chloride, phosphate, and bicarbonate. The transfer of ions across cellular membranes plays an essential role in vital bodily processes, including enzyme activation, protein metabolism, acid-base balance, nerve impulse transmission, and muscle contraction.