Ionization of neutral atoms can occur in several different ways. Compounds such as salts dissociate in solution into their ions, e.g., in solution sodium chloride exists as free Na+ and Cl - ions. Compounds that contain dissociable protons, or hydrogen ions, H+, or basic ions such as hydroxide ion, OH - , make acidic or basic solutions when they dissociate in water (see acids and bases; dissociation). Substances that ionize in solution are called electrolytes; those that do not ionize, like sugar and alcohol, are called nonelectrolytes. Ions in solution conduct electricity. If a positive electrode, or anode, and a negative electrode, or cathode, are inserted into such a solution, the ions are attracted to the electrode of opposite charge, and simultaneous currents of ions arise in opposite directions to one another. Nonelectrolytes do not conduct electricity.
Ionization can also be caused by the bombardment of matter with high-speed particles or other radiation. Ultraviolet radiation and low-energy X rays excite molecules in the upper atmosphere sufficiently to cause them to lose electrons and become ionized, giving rise to several different layers of ions in the earth's atmosphere (see ionosphere). A gas can be ionized by passing an electron current through it; the ionized gas then permits the passage of a much higher current. Heating to high temperatures also ionizes substances; certain salts yield ions in their melts as they do in solution.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.