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 ("ionic bond"). 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

    Today, of course, proton beams are routinely produced and accelerated to very high energies, in huge machines like the "Tevatron" south of Chicago and the accelerators of CERN (Europe's center for nuclear research) near Geneva. 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

• "From X-rays to Quarks" by Emilio Segre, a history of modern physics, including that of radioactivity and accelerators. Clearly and engagingly written by a Nobel prize winner, but not always easy for the non-scientist.
• "The God Particle" by Leon Lederman with Dick Teresi, the same story told by another Nobel laureate, but in a rather light-hearted and irreverent style.
• An elementary exposition on nuclear physics can be found in the sister-site of "Exploration of the Earth's Magnetosphere", titled "From Stargazers to Starships," in the section on solar energy and the following optional section on nuclear energy.

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