Definition of actinium series
: a radioactive series beginning with actinouranium, constituting the isotope of uranium of mass number 235, and ending with actinium D, constituting the nonradioactive isotope of lead of mass number 207: actinouranium, atomic number 92 (synonym uranium 235) → uranium Y, atomic number 90 (synonym thorium 231) → protactinium 231, atomic number 91 → actinium 227, atomic number 89 → radioactinium, atomic number 90 (synonym thorium 227) [or actinium K, atomic number 87 (synonym francium 223)] → actinium X, atomic number 88 (synonym radium 223) → actinon, atomic number 86 (synonym radon 219) → actinium A, atomic number 84 (synonym polonium 215) → actinium B, atomic number 82 (synonym lead 211) [or astatine 215, atomic number 85] → actinium C, atomic number 83 (synonym bismuth 211) → actinium C, atomic number 84 (synonym polonium 211) [or actinium C, atomic number 81 (synonym thallium 207)] → actinium D, atomic number 82 (synonym lead 207) — compare actinide series
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