 Heavier elements beyond the actinides were first proposed to exist as early as 1895, when the Danish chemist Hans Peter Jørgen Julius Thomsen predicted that thorium and uranium formed part of a 32-element period which would end at a chemically inactive element with atomic weight 292 (not far from the 294 known today for the first and only discovered isotope of oganesson).In 1913, the Swedish physicist Johannes Rydberg similarly predicted that the next noble gas after radon would have atomic number 118, and purely formally derived even heavier congeners of radon at "Z" = 168, 218, 290, 362, and 460, exactly where the Aufbau principle would predict them to be.Niels Bohr predicted in 1922 the electronic structure of this next noble gas at "Z" = 118, and suggested that the reason why elements beyond uranium were not seen in nature was because they were too unstable.