November - Harold Urey discovers deuterium, an isotope of hydrogen that contains one proton and one neutron.
February - James Chadwick discovers the neutron.
April - Max Born, James Franck and many other scientists are compelled to leave their posts at German universities because of their "Jewish physics." October - Leo Szilard recollects, "It occurred to me in October, 1933 that a chain reaction might be set up if an element could be found that would emit two neutrons when it swallowed one neutron." This idea becames a classified British patent in 1935 before fission was discovered.
Frédéric and Irène Joliot-Curie discover artificial radioactivity. Enrico Fermi irradiates uranium with neutrons. He believes he has produced the first transuranic element, but unknowingly achieves the world's first nuclear fission.
December - Fermi receives the Nobel Prize in Physics for the discovery of transuranic elements (actually fission of uranium) and departs for the United States. December 22 - Otto Hahn sends a paper to Lise Meitner containing experimental results that are interpreted by Meitner and nephew Otto Frisch as nuclear fission.
January 6 - Hahn and his assistant Fritz Strassmann publish their results. February 11 - Meitner and Frisch publish a theoretical interpretation of the Hahn-Strassmann results as nuclear fission. January to May - Many experiments on uranium fission are conducted by scientists in laboratories around the world. August 2 - Szilard, Eugene Wigner, and Edward Teller obtain a letter from Einstein on the possibility of a uranium weapon; President Roosevelt receives the letter on October 11, 1939 from Alexander Sachs, who was an unofficial adviser to the President. Hans Bethe recognizes that the fusion of hydrogen nuclei to form deuterium releases energy. He suggests that much of the energy output of the Sun results from fusion reactions. He would win the 1967 Nobel Prize in Physics for this effort.