The chemical element of atomic number 91, a radioactive metal of the actinide series, occurring in small amounts as a product of the natural decay of uranium.
- ‘The dates of discovery of these elements as 1789 for uranium, 1829 for thorium, 1899 for actinium and 1913 for protactinium.’
- ‘All isotopes of protactinium are radioactive.’
- ‘A new method of radioactive tracing involves the use of a new clock, based on the radioactive decay of uranium - 235 to protactinium - 231.’
- ‘A protactinium - 234 atom has 91 protons and 143 neutrons.’
- ‘Along the way, these disintegration series produce radioactive isotopes of protactinium, thorium, actinium, radium, francium, radon, astatine, polonium, bismuth, lead, thallium and mercury.’
Early 20th century: from proto- original, earlier + actinium, so named because one of its isotopes decays to form actinium.