noun
or uranium-radium series
: a radioactive series beginning with uranium I of mass number 238 and ending with radium G constituting the nonradioactive isotope of lead of mass number 206: uranium I, at. no. 92 → uranium X 1 at. no. 90 (syn. thorium 234) → uranium X 2 , at. no. 91 (syn. protactinium 234) → uranium II, at. no. 92 (syn. uranium 234) → ionium, at. no. 90 (syn. thorium 230) → radium 226, at. no. 88 → radon 222, at. no. 86 → radium A, at. no. 84 (syn. polonium 218) → radium B, at. no. 82 (syn. lead 214) [or astatine 218] → radium C, at. no. 83 (syn. bismuth 214) → radium C′, at. no. 84 (syn. polonium 214) [or radium C″, at. no. 81 (syn. thallium 210)] → radium D, at. no. 82 (syn. lead 210) → radium E, at. no. 83 (syn. bismuth 210) → radium F, at. no. 84 (syn. polonium 210) → radium G, at. no. 82 (syn. lead 206) — called also radium series