Q. What did Seaborg do?

        Details of answer;
      In the years before WWII, Seaborg (with McMillan, Livingood and Segré) had discovered numerous radioisotopes at the University of California, among them iodine-131, iron-59, cobalt-60 and technetium-99m, all of which have been used in medical imaging and radiation therapy from that day to this. Seaborg’s team then pursued the radiation emitting from the samples of neptunium-238 without success. Then, with Segré’s help, the Seaborg team discovered the hidden nature of plutonium, and the world changed.
      At 30 years of age, he joined Enrico Fermi at the Metallurgical Laboratory at the University of Chicago, where work commenced to beat the Germans to the atomic bomb. The efficiency of the powerful device was demonstrated on August 9, 1945, when the plutonium “Atom Bomb” devastated Nagasaki, Japan, helping to avoid door-to-door fighting in conquering the rest of the country, avoiding the loss of even greater numbers of Japanese - as well as American soldiers.
      Of all his honors, awards, discoveries and offices he admitted without hesitation that he was most proud of the Actinide Concept, a re-arrangement of the periodic table of the elements, which moved the heaviest elements to the bottom. "The heaviest elements at the time - thorium, protactinium and uranium - were up in the body of the table. In 1944 I moved them to where they are now, at the bottom, a separate row analogous to the rare-earth lanthanide elements." It was a somewhat audacious suggestion for a 32 years old scientist. "It was a very unpopular move at the time," Seaborg mused. "I was told it would ruin my scientific reputation." element 106 was later named seaborgium (Sg) in his honor.
      20 years later, Dr. Seaborg expressed approval of Roy Alexander’s patented 3-D form of the table, which moved the Rare Earths back to their correct position by employing a loop from the locations of Actinium and Lanthanum.
      The number of elements in the f-block is either 14 or 15 elements long. Firm believers exist of both numbers, and those of 14 are divided as to whether lanthanum or lutetium should remain with the transition metals.

        Resources;
mooni.fccj.org, AllPeriodicTables.com, av8n.com