protactinium

Meanings

1. 1
   noun

   the chemical element with symbol Pa and atomic number 91, a dense, silvery, highly radioactive actinide metal with no stable isotopes

   Protactinium-231 occurs in trace amounts in uranium ores and decays to actinium-227.

From Greek protos ("first, before") + actinium, because the long-lived isotope protactinium-231 decays into actinium-227: it stands before, or is the parent of, actinium in that decay series. The original name protoactinium was later shortened to protactinium.

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