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English definition of “atom”

atom

noun [C]     /ˈæt.əm/ US  /ˈæt̬-/
B2 the smallest unit of any chemical element, consisting of a positive nucleus surrounded by negative electrons. Atoms can combine to form a molecule: A molecule of carbon dioxide (CO2) has one carbon atom and two oxygen atoms.figurative He hasn't an atom of sense (= he has no sense), that boy.Atoms, molecules and sub-atomic particles
(Definition of atom noun from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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