Introduction to Ions and Ionization
Ions and Ionization, An ion is an electrically charged atom or group of atoms. A normal atom is electrically neutral, because it has the same number of electrons (particles bearing a negative charge) as protons (particles bearing a positive charge). The atom becomes a negative ion, or anion, upon gaining one or more electrons. The atom becomes a positive ion, or cation, when it loses one or more electrons. A molecule or a radical (two or more atoms that act as a unit in forming a molecule) may also become an ion.
Ionization is the process by which an electrically neutral atom, molecule, or radical loses or gains one or more electrons and becomes an ion. Ionization can occur in gases, liquids, or solids. Physicists often call a highly ionized gas a plasma.
Metals tend to form positive ions, nonmetals negative ions. An amphoteric ion carries both a positive and a negative charge, usually at opposite ends of a long chain structure. Most amphoteric ions are organic (contain carbon). Ions of amino acids are typical amphoteric ions.
How Ions Are Formed
Ions are formed mainly by a chemical process called dissociation and by physical processes involving collisions.
DissociationIn this process, a substance separates into oppositely charged ions (that is, both positive and negative ions) when dissolved in water or some other solvent. Substances that become ionized in this manner are called electrolytes and include acids, bases, and salts. Dissolved electrolytes are generally good conductors of electricity.
CollisionsWhen a fast-moving electron, ion, or other particle collides with an atom or molecule, the atom or molecule may be stripped of one or more electrons. In photoionization, an atom or molecule ejects an electron when it is struck by certain kinds of electromagnetic radiation, such as X rays or ultraviolet radiation. High-energy electromagnetic radiation and high-speed particles capable of ionizing atoms are often referred to as ionizing radiation.
Ionization resulting from collisions generally forms positive ions and free electrons; the electrons may, in turn, combine with other atoms to form negative ions. In an electric discharge such as lightning or the electric arc in a fluorescent lamp, atoms are ionized by collisions with free electrons accelerated by a high voltage. Atoms of a very hot gas move so rapidly that they can become ionized through collisions with each other. An example of photoionization is the ionization produced by ultraviolet radiation from the sun in the upper region of the earth's atmosphere called the ionosphere.
Why Ions Are Important
The great attraction that ions of opposite charges exert on each other makes ions very reactive chemically. This attraction is also the basis for one kind of chemical bond—the ionic bond, most commonly found in inorganic compounds. Many salts and other crystalline compounds are composed mainly of ions (rather than molecules), arranged in lattice-like structures. However, the chemical bonds in such crystalline compounds are never completely ionic.
Ions are especially important in electrochemistry, the study of the relation between chemical change and electrical energy. Svante A. Arrhenius was the first to suggest, in the 1880's, that certain solutions conduct electricity because they contain charged atoms, or ions. The movement of ions forms the electric current in dry cells, storage batteries, and most other electrical systems that do not use metals to conduct the current. In electrolysis, electrical energy is used to produce chemical changes by means of ions.
The study of ions is important in chemistry, physics, biology, and geology. In industry, several processes involving ions are used, including electroplating, anodizing, and arc welding. The ionization of gases is used in the operation of such devices as electric discharge lamps and Geiger counters.
Living tissues contain large concentrations of ions. Among the most important are ions of sodium, calcium, and potassium. The ions must be present in certain proportions for the normal functioning of cells. Ions are involved in the transmission of impulses along nerve fibers and in the contraction of muscle fibers.
Ion ExchangeIon exchange is a reversible reaction in which an insoluble substance (the ion exchanger) exchanges ions with a solution by substituting one kind of ion for another with the same charge. Plants gain nutrient ions from the soil in this way. Ion exchangers are able to extract unwanted ions from a solution, or to inactivate them by adding ions that will unite with the unwanted ions.
Ion exchange is used to purify and separate many chemicals, including radioactive isotopes and amino acids, and to treat ulcers, edema, and other diseases. Zeolites are mineral ion exchangers commonly used in water softening.
Chemical Symbols For Ions
The chemical symbol for an ion consists of: (1) the symbol or symbols for elements it contains; and (2) a sign showing the number of electrons it must lose or gain to balance its protons electrically. (This number equals the valence number, showing how many electrons a substance may lose, gain, or share in forming a chemical bond.) Each electron to be gained is indicated by a plus sign, which denotes a positive ion; each electron to be lost, by a minus sign, which denotes a negative ion.
For example, the hydrogen ion H+ must gain one electron to balance the positive charge of its proton. The zinc ion Zn++ (or Zn2+) must gain two electrons to achieve a neutral structure. The hydroxide ion OH-, with one excess electron, is negative.