1930

Schrödinger views electrons as continuous clouds and introduces "wave mechanics" as a mathematical model of the atom.

1931

Albert Einstein urges all scientists to refuse military work. Harold C. Urey of the United States and associates discover deuterium (heavy hydrogen) which is present (0.014%) in all natural hydrogen compounds including water. John D. Crockcroft of Great Britain develops high-voltage apparatus for atomic transmutation. 

1932

James Chadwick proves the existence of neutrons, using alpha particles striking a beryllium foil. He determines their mass by measuring the recoil tracks of known atoms of the rarified gas in his cloud chamber. 

John Cockcroft and E. T. S. Walton of Great Britain split the atom on a linear accelerator built at Ernest Rutherford's Cavendish Laboratory at Cambridge University.  Their experiment proves Albert Einstein's theory of relativity.

Leo Szilard reads H.G. Wells' novel, The World Set Free, in which Wells prophesizes an atomic war in which the major cities of the world are destroyed

August 2
American experimentalist Carl Anderson discovers a new particle the "positron." It is an electron with a positive instead of negative charge.

1933

January 30
Adolph Hitler becomes Chancellor of Germany.

March 23
Following the Reichstag fire and subsequent suspension of constitutional liberties, Reichstag voluntarily gives over its powers to Hitler's cabinet.

April 7
Third Reich promulgates its first anti-Jewish ordinance.

September 12
Leo Szilard, a Hungarian physicist who took refuge in London from Nazi Germany, reads about a speech in which Lord Rutherford ridiculed the idea of using the transformation of atoms as a source of power. Szilard realizes that, "if we could find an element which is split by neutrons, and which would emit two neutrons when it absorbs one neutron, such an element could sustain a nuclear chain reaction." 

1934

Frederic and Irene Joliot-Curie of France discover artificial radioactivity, i.e. the radioactivity of atoms produced in transmutation experiments.  Enrico Fermi of Italy irradiates uranium with neutrons. He believes he has produced the first transuranic element, but unknowingly achieves the world's first nuclear fission.

June 28 and July 4
Leo Szilard files for patent amendments for "the liberation of nuclear energy for power production and other purposes through nuclear 'transmutation.' " He proposes a "chain reaction" for the first time.

1935

April 9
Leo Szilard files patent amendment identifiying uranium and bromine as "examples for elements from which neutrons can liberate multiple neutrons..."

When Szilard learns that the only way for his patents on liberating nuclear energy can be kept secret is to assign them to an agency of the British government he offers them to the British War Office.

October 8
The British War turns down Leo Szilard's offer to give them his patents on nuclear energy stating, "There appears to be no reason to keep the specification secret so far as the War Department is concerned."

1936

February
Leo Szilard's offer to turn over patents on nuclear energy, in order to keep them secret, is accepted by British Admiralty.

1938

Otto Hahn and Fritz Strassmann of Germany split the uranium atom by bombarding it with neutrons and show that the elements barium and krypton are formed.

Lisa Meitner conducts experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction.

Werner von Braun is appointed the technical director of the German rocket research center at Peenemünde. 

1939

Lise Meitner and Otto Frisch of Austria announce theory of nuclear fission. Frederic Joliot demonstrates the possibility of splitting the atom of uranium isotope 235. Hans A. Bethe, a German-born physicist, recognizes that the fusion of hydrogen nuclei to form deuterium releases energy. He suggests that much of the energy output of the Sun and other stars results from energy-releasing fusion reactions in which four hydrogen nuclei unite and form one helium nucleus.

January 13
Otto Frisch detects fission fragments in an ionization chamber. He adopts the term "fission."

January 25
First experimental fission in U.S. takes place at Columbia University.

January 26
Niels Bohr announces discovery of fission at a conference in theoretical physics at George Washington University in U.S.

January 29
Upon hearing of discovery of fission, Robert Oppenheimer immediately grasps the possibility of atomic bombs.

July 3
Leo Szilard writes to Enrico Fermi describing the concept (uranium lattice in carbon) for creating a chain reaction.

August 2
Albert Einstein's first letter to President Franklin Roosevelt leads to the formation of the Committee on Uranium. The letter, originally drafted by Leo Szilard states "that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the part of the Administration." After the bombing of Hiroshima, Einstein states, "I could burn my fingers that I wrote that first letter to Roosevelt."

September 1
Germany invades Poland. World War II begins.