noun
: a radioactive series beginning with thorium of mass number 232 and ending with thorium D which is the nonradioactive isotope of lead of mass number 208: thorium, at. no. 90 → mesothorium 1, at. no. 88 (syn. radium 228) → mesothorium 2, at. no. 89 (syn. actinium 228) → radiothorium, at. no. 90 (syn. thorium 228) → thorium X, at. no. 88, (syn. radium 224) → thoron, at. no. 86 (syn radon 220) → thorium A, at. no. 84 (syn. polonium 216) → thorium B, at. no. 82 (syn. lead 212) [or astatine 216] → thorium C, at. no. 83 (syn. bismuth 212) → thorium C′, at. no. 84 (syn. polonium 212) [or thorium C″, at. no. 81 (syn. thallium 208)] → thorium D, at. no. 82 (syn. lead 208)