ion, 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.

hydrogen-ion concentration: see pH.

field-ion microscope: see microscope.

complex ion, charged molecular aggregate (see ion), consisting of a metallic atom or ion to which is attached one or more electron-donating molecules. In some complex ions, such as sulfate, SO₄^-2, the atoms are so tightly bound together that they act as a single unit. Many complex ions, however, such as tetrammine zinc (II), Zn(NH₃)₄⁺², are only loosely aggregated and tend to dissociate in a water solution until an equilibrium is established between the complex ion and its components (see chemical equilibrium). Such complex ions, or coordinated complexes as they are also called, generally consist of a positively charged central metal atom or ion, like the zinc in tetramine zinc, surrounded by electron-donating, or basic, groups called ligands; in the tetrammine zinc complex, the NH₃ groups are the ligands. The number of bonds connecting the ligands to the central atom or ion is its coordination number, or ligancy. Transition metals (see transition elements) are especially suited for forming complex ions because they have filled or partially filled electron orbitals that can participate in bonding the ligands to the metal. The bonding holding the ligands to the central atom or ion is similar to covalent bonding between atoms but is more complex (see chemical bond). All the ligands surrounding the central ion need not be the same, and some positions can be occupied by solvent molecules. Because ligands remain in a fixed position around a central atom or ion, in many complexes different isomers, or arrangements, of the ligand groups are possible. When there are four or more ligands around a central atom, different stereoisomers, or spatial configurations, are possible (see stereochemistry). Many complex ions are colored; the specific color of a complex depends on both the central atom or ion and the ligands. For example, when cobaltous chloride is dissolved in water, a pale pink solution, sometimes called invisible ink, results because of the presence of the hydrated cobaltous ion, Co(H₂O)₆⁺²; this solution does not show up well on paper, but if the paper is heated to drive the water off, visibility improves because of the formation of a blue tetrachlorocobalt (II)^-2 complex. Some of the more important complex ions are vitamin B₁₂, chlorophyll, and the heme component of hemoglobin, in which the central metal ions are cobalt, magnesium, and iron, respectively, and the ligands are complex organic systems. Many enzymes contain a metal ion about which parts of the protein are coordinated.

common-ion effect, decrease in solubility of an ionic salt, i.e., one that dissociates in solution into its ions, caused by the presence in solution of another solute that contains one of the same ions as the salt. The common-ion effect is an example of chemical equilibrium. For example, silver chloride, AgCl, is a slightly soluble salt that in solution dissociates into the ions Ag⁺ and Cl^-, the equilibrium state being represented by the equation AgCl_solid ⇌Ag⁺+Cl^-. According to Le Châtelier's principle, when a stress is placed on a system in equilibrium, the system responds by tending to reduce that stress. In the system taken as an example, if another solute containing one of those ions is added, e.g., sodium chloride, NaCl, which supplies Cl^- ions, the solubility equilibrium of the solution will be shifted to remove more Cl^- from the solution, so that at the new equilibrium point there will be fewer Ag⁺ ions in solution and more AgCl precipitated out as a solid.

Maurer, Ion Gheorghe, 1902-2000, Romanian statesman. A lawyer, he defended Romanian Communists in the 1930s and later became (1945) a member of the central committee of the Communist party. In the postwar decades he was minister of economic affairs (1946-47), minister of industry and trade (1947-48), foreign minister (1957-58), and chairman of the presidium of the national assembly (1958-61). In 1961 he became chairman of the council of ministers, or premier. An intimate of Nicolae Ceauşescu, Maurer supported Romania's nationalist and independent policy. He curtailed his activities after a serious car accident in 1970 and resigned as premier in 1974.

Ion: see Creusa 1.

Caragiale, Ion Luca, 1853-1912, Romanian playwright and author. Romania's foremost dramatist, his works sharply satirized Romanian society. His masterpiece, A Lost Letter (1884), describes a provincial government election won by a blackmailer. Other plays include Carnival Adventures (1885) and False Accusation (1889), a tragedy. He also wrote short stories and novels.

Antonescu, Ion, 1882-1946, Romanian marshal and dictator. He served in World War I and later became chief of staff, but he fell into disfavor with King Carol II because of his pro-Nazi attitude and his suspected intrigues with the Iron Guard. In World War II, on Sept. 4-5, 1940, Carol, threatened with revolution and German intervention, appointed Antonescu premier with dictatorial powers. On Sept. 6, Antonescu forced the king to abdicate in favor of Carol's son, Michael. In Nov., 1940, Romania joined the Axis Powers, and Antonescu gave Adolf Hitler virtual control over Romanian economy and foreign policy, tolerated violent pogroms against the Jews, and declared (June 22, 1941) a "holy war" on the Soviet Union. With two Soviet armies deep in Romania, King Michael in Aug., 1944, had Antonescu and his cabinet arrested in a dramatic coup. Antonescu was tried (1946) for war crimes and executed.

Any of a wide variety of synthetic polymers containing positively or negatively charged sites that can interact with or bind to an ion of opposite charge from a surrounding solution. Light, porous solids in granules, beads, or sheets, they absorb the solution and swell as they attract the target ions; when exhausted, they are removed from use and regenerated by an inexpensive brine or carbonate solution. A solid support of styrene-divinylbenzene copolymer to which are attached sulfonic or carboxylic acid groups is often used to attract and exchange cations (e.g., ions of hydrogen or metals). Quaternary ammonium groups on the solid matrix are used to attract anions (e.g., ions of chlorine). Industrially, the resins are used to soften hard water, purify sugar, and concentrate valuable elements (gold, silver, uranium) from their ores. In the laboratory they are used to separate and concentrate substances and sometimes as catalysts. Zeolites are minerals with ion-exchange properties.

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Atom or group of atoms with one or more positive or negative electric charges. Positively charged ions are cations, negatively charged ones anions. Ions are formed when electrons are added to or removed from neutral molecules or other ions, as sodium (Na) and chlorine (Cl) atoms react to form Na⁺ and Cl⁻; when ions combine with other particles, as hydrogen cations (H⁺) and ammonia (NH₃) combine to form ammonium cations (NH₄⁺); or when a covalent bond between two atoms is ruptured in such a way that the resulting particles are charged, as water (H₂O) dissociates (see dissociation) into hydrogen and hydroxide ions (H⁺ and OH⁻). Many crystalline substances (see crystal) are composed of ions held in regular geometric patterns by the attraction of oppositely charged particles for each other. Ions migrate to the electrode of opposite charge in an electric field and are the conductors of current in electrolytic cells (see electrolysis). Compounds that form ions are called electrolytes. Ions are also formed in gases when heated to very high temperatures or when an electrical discharge passes through them (see plasma).

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(born June 15, 1882, Pitessubcommati, Rom.—died June 1, 1946, near Jilava) Romanian marshal. He served in World War I and later became chief of staff (1934) and minister of defense (1937). Appointed prime minister in 1940, he established a fascist dictatorship and openly supported the Axis Powers. Initially he won broad support for his domestic reforms and declaration of war against the Soviet Union (1941), but that support eroded. His regime was overthrown in 1944, and he was later executed as a war criminal.

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