Promethium

A group discovered element number 61, named promethium, with its most stable isotope having a half-life of 17.7 years. This element filled the gap in the periodic table below uranium.
 Charles Du-Bois Coryell United States 1945 Promethium

A group discovered element number 61, named promethium, with its most stable isotope having a half-life of 17.7 years. This element filled the gap in the periodic table below uranium.