atomic theory has 3 distinct synonyms spread across 1 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a theory of the structure of the atom
(chemistry) any theory in which all matter is composed of tiny discrete finite indivisible indestructible particles
“the ancient Greek philosophers Democritus and Epicurus held atomic theories of the universe”
Synonyms, antonyms and the broader and narrower relations for atomic theory come from Princeton University’s WordNet. Full sources and licences.