Ion

An ion is an atom or molecule that has a net electric charge due to the loss or gain of one or more electrons. If electrons are removed, the species becomes a positively charged ion, or cation; if electrons are gained, it becomes a negatively charged ion, or anion. Monoatomic ions include Na+ and Cl−; polyatomic ions include NH4+, NO3−, and SO4^2−.

Ionization can occur by chemical reactions (redox), heating, photoionization, or electrical discharge. In solution, ions are surrounded by solvent molecules, a process called solvation, which affects their mobility and reactivity. Ionic compounds such as sodium chloride form crystal lattices in the solid state and dissociate into ions in water.

Ions play essential roles in biology, chemistry, and technology. In living organisms, ions such as Na+, K+, Ca2+, and Cl− regulate nerve signaling, muscle activity, and osmotic balance. In industrial and analytical contexts, ions are key charge carriers in electrolytes and in electrochemical cells, and they move through membranes via ion channels.

Historically, the term ion was coined by Wilhelm Ostwald in 1891, and the idea of ions was developed through the work of Arrhenius on electrolytic dissociation, which helped explain conductivity in solutions and the behavior of salts.