Protactinium

By this point most new "elements" turned out to be isotopes of existing ones, but protactinium was an exception, highly radioactive and spontaneously disintegrates into actinium. This element filled one of the seven gaps in the periodic table identified by Moseley.
 Otto Hahn, Lise Meitner Germany, Austria 1917 Protactinium

By this point most new "elements" turned out to be isotopes of existing ones, but protactinium was an exception, highly radioactive and spontaneously disintegrates into actinium. This element filled one of the seven gaps in the periodic table identified by Moseley.