WordNet records no synonym and no antonym for free electron. It is the only member of its synset, 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.
electron that is not attached to an atom or ion or molecule but is free to move under the influence of an electric field
Synonyms, antonyms and the broader and narrower relations for free electron come from Princeton University’s WordNet. Full sources and licences.