| protoactinium | <chemical> Protactinium. A radioactive element of the actinide group of metals. It has the atomic symbol pa, atomic number 91, and atomic weight 231. It decays by alpha-emission. Chemical name: Protactinium (12 Dec 1998) |
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| protoactinium | a short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into lead |
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