#8H.     Positive Ions--History

Ions in chemistry

The notion of ions first arose in chemistry. In the 19th century it was well known that water in which salts were dissolved (or acids, or bases) conducted electricity, and that an electric current could separate such dissolved materials into their components. Faraday formulated the laws of such processes.

But how, and why?

The answer was given in 1884 by Svante Arrhenius (1859-1927), a many-talented Swede who received the 1903 Nobel prize for chemistry and who (among his many achievements) first suggested the "greenhouse effect." Arrhenius proposed that when a compound like table salt NaCl (sodium chloride) was dissolved in water, it broke up into electrically charged "ions" (Greek for "the ones that move") Na+ and Cl-. Electric forces made Na+ ions move in one direction, Cl- ions in the opposite one, and that was how the electric current was carried.

Although at first this seemed like a strange idea, today it is quite well understood. Many chemical molecules are formed when atoms share electrons, but molecules such as those of NaCl are different. There, the sodium atom (Na) gives up an electron to the chlorine (Cl), creating ions Na+ and Cl-, which in solid salt are held together by their electric attraction. Water, however, greatly weakens that attraction (on a microscopic scale), allowing the ions to drift free whenever salt is dissolved in water, and allowing the water to conduct electricity.

Ions in a plasma

In plasma discharges in rarefied gases (like those in fluorescent tubes) ions are also produced. J.J. Thomson, who discovered the electron, later (1907-11) produced narrow beams of protons in a near-vacuum and studied their reaction to magnetic and electric forces.

Today, of course, proton beams are routinely produced and accelerated to very high energies, in huge machines like the "Tevatron" south of Chicago. Some of them are allowed to smash into targets, to study the structure of matter and produce a variety of "new" particles. Another "cleaner" mode of studying them is to cause a head-on collision between a beam of protons and another one of antiprotons, "antimatter" particles resembling protons but with a negative charge. The proton-antiproton collision is cleaner because it only involves two relatively simple particles, but the antiprotons must first be produced by some other high-energy collisions, since they do not usually exist in nature.

Ions emitted by radioactivity

Radioactivity was discovered in 1895, when it was found that heavy elements such as uranium emitted "rays" which could ionize air and fog photographic film. In 1898 Ernest Rutherford noted that the radiation seemed to contain two electrically charged components of opposite signs, steered by a magnet in opposite directions--positive "alpha rays" and negative "beta rays."

Ultimately beta rays were identified as electrons and "alpha particles" as completely ionized helium nuclei; a third component, "gamma rays" unaffected by magnets, turned out to be related to light and X-rays. For his work on radioactivity, Rutherford was awarded a Nobel prize in 1908.

Alpha particles

It was later found that heavy nuclei such as uranium were made unstable by the large number of protons they contained (being all positive, the protons repel each other). Such nuclei therefore expelled some of their extra protons in the form of alpha particles, which (as noted) form an extremely stable configuration. Alpha particles released by rocks underground ultimately find electrons and settle down as ordinary helium atoms, dispersed in the Earth's crust. Some of this helium finds its way into natural gas, and it can be extracted from there for a variety of uses. Thus practically all of the helium gas used in toy balloons once started out as alpha radiation!

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Last updated March 13, 1999