b has 12 distinct synonyms spread across 6 of its 7 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.
aerobic rod-shaped spore-producing bacterium; often occurring in chainlike formations; found primarily in soil
originally thought to be a single vitamin but now separated into several B vitamins
a trivalent metalloid element; occurs both in a hard black crystal and in the form of a yellow or brown powder
Synonyms, antonyms and the broader and narrower relations for b come from Princeton University’s WordNet. Full sources and licences.