ION, in Chemistry

atom or group of atoms having a net electric charge.

Positive and Negative Electric Charges

A neutral atom or group of atoms becomes an ion by gaining or losing one or more electrons or protons. Since the electron and proton have equal but opposite unit charges, the charge of an ion is always expressed as a whole number of unit charges and is either positive or negative. A simple ion consists of only one charged atom; a complex ion consists of an aggregate of atoms with a net charge. If an atom or group loses electrons or gains protons, it will have a net positive charge and is called a cation. If an atom or group gains electrons or loses protons, it will have a net negative charge and is called an anion.

Since ordinary matter is electrically neutral, ions normally exist as groups of cations and anions such that the sum total of positive and negative charges is zero. In common table salt, or sodium chloride, NaCl, the sodium cations, Na⁺, are neutralized by chlorine anions, Cl⁻. In the salt sodium carbonate, Na₂CO₃, two sodium cations are needed to neutralize each carbonate anion, CO₃⁻², because its charge is twice that of the sodium ion.

Ionization of Neutral Atoms

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.

Applications of Ionization

Ionization has many applications. Vapor lamps and fluorescent lamps take advantage of the light given off when positive ions recombine with electrons. Because of their electric charge the movement of ions can be controlled by electrostatic and magnetic fields. Particle accelerators, or atom smashers, use both fields to accelerate and aim electrons and hydrogen and helium ions. The mass spectrometer utilizes ionization to determine molecular weights and structures. High-energy electrons are used to ionize a molecule and break it up into fragment ions. The ratio of mass to charge for each fragment is determined by its behavior in electric and magnetic fields. The ratio of mass to charge of the parent ion gives the molecular weight directly, and the fragmentation pattern gives clues to the molecular structures.

In ion-exchange reactions a specially prepared insoluble resin with attached dissociable ions is packed into a column. When a solution is passed through the column, ions from the solution are exchanged with ions on the resin (see chromatography). Water softeners use the mineral zeolite, a natural ion-exchange resin; sodium ions from the zeolite are exchanged for metal ions from the insoluble salt that makes the water hard, converting it to a soluble salt. Ion-permeable membranes allow some ions to pass through more readily than others; some membranes of the human nervous system are selectively permeable to the ions sodium and potassium.

Engineers have developed experimental ion propulsion engines that propel rockets by ejecting high-speed ions; most other rocket engines eject combustion products. Although an ion engine does not develop enough thrust to launch a rocket into earth orbit, it is considered practical for propelling one through interplanetary space on long-distance trips, e.g., between the earth and Jupiter. If left running for long periods of time on such a trip, the ion engine would gradually accelerate the rocket to immense speeds.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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