Meaning of ATOM in English

[noun] [C] - 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 (= not even the smallest bit) of sense, that boy.(slightly dated) An atom bomb (also atomic bomb) is a bomb that uses the explosive power that results from breaking the atom.Over 200 000 Japanese people were killed or wounded when two atom bombs were dropped on Hiroshima and Nagasaki in 1945.

Cambridge English vocab.      Кембриджский английский словарь.