WordNet records no synonym and no antonym for atrium. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
any chamber that is connected to other chambers or passageways (especially one of the two upper chambers of the heart)
the central area in a building; open to the sky
Synonyms, antonyms and the broader and narrower relations for atrium come from Princeton University’s WordNet. Full sources and licences.