Definition: Formerly considered the ultimate particle of an element, believed to be as indivisible as its name indicates. Discovery of radioactivity demonstrated the existence of subatomic particles, notably protons, neutrons, and electrons, the first two making up most of the mass of the atomic nucleus. It is now known that subatomic particles are further classified into hadrons, leptons, and quarks.
[G. atomos, indivisible, uncut]
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Examples: glitazone, GI cocktail, etc.