/ Thesaurus / A Synonyms of atomic number 91 Find synonyms for: Noun1. protactinium, protoactinium, Pa, atomic number 91, metallic element, metalusage: a short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into leadWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: eccentric Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language