mercury has 4 distinct synonyms spread across 1 of its 4 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 heavy silvery toxic univalent and bivalent metallic element; the only metal that is liquid at ordinary temperatures
(Roman mythology) messenger of Jupiter and god of commerce; counterpart of Greek Hermes
Synonyms, antonyms and the broader and narrower relations for mercury come from Princeton University’s WordNet. Full sources and licences.
the smallest planet and the nearest to the sun
temperature measured by a mercury thermometer
“the mercury was falling rapidly”