SCIENCE A TO Z

Every Sunday on this page the Sun-Times is presentingdefinitions of modern scientific terms, as compiled by the LondonDaily Telegraph and the Science Museum of London.

Antimatter: It was British physicist Paul Dirac (1902-84)who first mathematically predicted the existence of antimatter. Hemade the discovery in the '20s, when he attempted to marry two bigtheories of his day - Einstein's special theory of relativity andquantum physics.

He came up with the "Dirac equation," which predicted thatan antielectron must exist, one with the same mass as an electron butwith an opposite charge. When this particle - called the positron -met an electron, both would be annihilated in a burst of gamma rays.So, if you tried to shake hands with your antimatter alter ego, youwould annihilate each other in a flash of radiation.Carl Anderson independently discovered a particle with thesame mass as the electron but with the opposite charge in 1932.Anderson had discovered Dirac's antielectron, but at the time neitherknew of the other's work.Atoms: It hardly seems possible, but a century ago manyscientists did not believe in the existence of atoms.Modern atomic theory dates to the English natural philosopherJohn Dalton (1766-1844), though it was in 1905 that the matter wassettled by Einstein's explanation of Brownian motion - the dance ofsuspended particles - in terms of collisions with water molecules.Atoms were seen by the Greeks as indivisible units ofmatter. Now they are seen as the smallest units that bear thechemical characteristics of an element, whether hydrogen or uranium.Around a million atoms would fit on a period (.) 1millimeter across. They are mostly empty space: The nucleus, wheremost mass resides, is 100,000 times smaller than the atom. Each atomconsists of a positively charged nucleus orbited by a mist ofnegative charge. The mist consists of one or more electrons; thenucleus consists of positively charged protons, and neutrons, whichhave no charge.The reason we talk about a "mist" is due to the odd pictureof the atom to emerge from quantum mechanics. Physicists think ofelectrons as fuzzy clouds surrounding the nucleus rather than planetsorbiting the sun.A complete atom has the same number of electrons andprotons, balancing overall charge. Variations in the number ofneutrons produce different isotopes, which are chemically similar(chemical properties depend on the numbers of electrons) but weighdifferent amounts.Uranium-238 has 92 protons and 146 neutrons. Uranium-235 hasthree fewer neutrons, an unstable arrangement that tends to fallapart, losing mass that is converted into energy. This is exploitedin nuclear bombs.