The discovery of radioactivity by Henri Becquerel in 1896 opened a new field of research for chemists. An immediate and obvious question had to do with the materials that produced the radioactivity that Becquerel observed. In July, 1898, Marie Curie announced the discovery of polonium, which, along with uranium, produced some of the radiation observed by Becquerel. Five months later, she identified radium, a second new element that was also radioactive. Over the next two decades, claims for nearly 50 new radioactive elements were put forward.

These claims presented problems for chemists. Only a handful of empty spaces remained in the periodic table for new elements. They did not know how these new radioactive elements would be placed in the table?

Some possible answers to this dilemma appeared quite early. Chemists noted that many of the new "elements" seemed to be chemically indistinguishable from each other or from other known elements...