 Seaborg was the principal or co @-@ discoverer of ten elements : plutonium , americium , curium , berkelium , californium , einsteinium , fermium , mendelevium , nobelium and element 106 , which , while he was still living , was named seaborgium in his honor . He also discovered more than 100 atomic isotopes and is credited with important contributions to the chemistry of plutonium , originally as part of the Manhattan Project where he developed the extraction process used to isolate the plutonium fuel for the second atomic bomb . Early in his career , he was a pioneer in nuclear medicine and discovered isotopes of elements with important applications in the diagnosis and treatment of diseases , most notably iodine @-@ 131 , which is used in the treatment of thyroid disease . In addition to his theoretical work in the development of the actinide concept , which placed the actinide series beneath the lanthanide series on the periodic table , he postulated the existence of super @-@ heavy elements in the transactinide and superactinide series . 
