a toxic radioactive metallic element that occurs in uranium ores and is produced by neutron irradiation of thorium. Symbol: Pa; atomic no: 91; half-life of the most stable isotope, 231Pa: 32 500 years; valency: 4 or 5; relative density: 15.37 (calc.); melting pt: 1572°C
Former name: protoactinium
protactinium in American English
(ˌproʊtækˈtɪniəm)
noun
a rare, radioactive, metallic chemical element, one of the actinides, found in pitchblende: symbol, Pa; at. no., 91