Roentgenium 's a chemical element in the periodic table that's the symbol Rg and atomic number 111. It's a synthetic element whose most stable known isotope has a mass of 280 amu and a half-life of 3.6s. Official discovery Roentgenium was officially discovered by Peter Armbruster, Gottfried Münzenberg, and their team working at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany on December 8, 1994. Only three atoms of it were observed (all 272Rg), by the cold fusion… (More on Roentgenium)