- mesothorium⇄mesothorium, noun.
either of two radioactive isotopes, mesothorium I (?at.no.: 88; ?half-life: 6.7 years), an isotope of radium, formed from thorium and yielding mesothorium II (?at.no.: 89; ?half-life: 6.13 hours), an isotope of actinium. Both isot