| thorium |
a rare, heavy, gray metal, atomic number, 90; atomic weight, 232.038. The naturally occurring radioactive isotope 232Th has a half-life of 1.4 × 1010 years and is the parent element of a radioactive disintegration series.
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| thorium e. |
one member of the radioactive series derived from thorium. It is produced from thorium X, has an atomic weight of 220, its atomic number is 86, and by the loss of alpha particles it changes into thorium A.
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