The process by which a neutral or uncharged atom or molecule (of gases, liquids, solids) acquire a charge, thus becoming an ion.
When a substance dissolved in a liquid (or the liquid itself) undergoes ionization, electrolytic ion pairs of opposite charge are produced at once. A substance in the gaseous state requires a source of energy for ionization. When a substance in gaseous state is ionized, usually by radiation, only positively charged molecules or ions are formed as the result of the loss of one or more electrons. Both the positive ions and electrons are free to cause further ionization of the gas and produce both positive and negative ions. In all cases, the charged particles and ions are subject to motion or deflection in electric fields. Their motion constitutes a flow of electric current.