the news of the discovery of fission was brought to america by bohr in january 1939. bohr and john a. wheeler set to work applying the liquid drop model developed by bohr and fritz kalckar to explain the mechanism of nuclear fission. george placzek, who was skeptical about the whole idea of fission, challenged bohr to explain why uranium seemed to fission with both very fast and very slow neutrons. bohr had an epiphany that the fission at low energies was due to the uranium-235 isotope, while at high energies it was mainly due to the more abundant uranium-238 isotope. the former makes up just 0.7% of natural uranium; while that latter accounts for 99.3%. on 16 april, bohr, placzek, wheeler, eugene wigner and leon rosenfeld discussed whether it would be possible to use a nuclear chain reaction to make an atomic bomb, and concluded that it was not. bohr observed that "it would take the entire efforts of a country to make a bomb."