| 1800 | Dalton firmly establishes atomic theory of matter. |
| 1890-1900 | Thomson’s experiments with cathode rays prove the existence of electrons. Atoms are found to contain negative electrons and positive electric charge. Becquerel discovers unstable (radioactive) atoms. |
| 1905 | Einstein postulates the equivalence of mass and energy. |
| 1911 | Rutherford recognizes nucleus. |
| 1919 | Rutherford achieves transmutation of one stable chemical element (nitrogen) into another (oxygen). |
| 1920-1925 | Improved mass spectrographs show that changes in mass per nuclear particle accompanying transmutation account for energy released by nucleus. |
| 1932 | Chadwick identifies neutrons. |
| 1939 | Discovery of uranium fission by German scientists. |
| 1940 | Discovery of neptunium by Edwin M. McMillan and Philip H. Abelson and of plutonium by Glenn T. Seaborg and associates at the University of California. |
| 1942 | Achievement of first self-sustaining nuclear reaction, University of Chicago. |
| 1945 | First successful test of an atomic device, near Alamagordo, New Mexico, followed by the dropping of atomic bombs on Hiroshima and Nagasaki, Japan. |
| 1946 | U. S. Atomic Energy Commission established by Act of Congress. |
| | First shipment of radioisotopes from Oak Ridge goes to hospital in St. Louis, Missouri. |
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| 1951 | First significant amount of electricity (100 kilowatts) produced from atomic energy at testing station in Idaho. |
| 1952 | First detonation of a thermonuclear bomb, Eniwetok Atoll, Pacific Ocean. |
| 1953 | President Eisenhower announces U. S. Atoms-for-Peace program and proposes establishment of an international atomic energy agency. |
| 1954 | First nuclear-powered submarine, Nautilus, commissioned. |
| 1955 | First United Nations International Conference on Peaceful Uses of Atomic Energy held in Geneva, Switzerland. |
| 1957 | First commercial use of power from a civilian reactor takes place in California. |
| | Shippingport Atomic Power Plant in Pennsylvania reaches full power of 60,000 kilowatts. |
| | International Atomic Energy Agency formally established. |
| 1959 | First nuclear-powered merchant ship, the Savannah, launched at Camden, New Jersey. |
| | Commissioning of first nuclear-powered Polaris missile-launching submarine George Washington. |
| 1961 | A radioisotope-powered electric power generator placed in orbit, the first use of nuclear power in space. |
| 1962 | Nuclear power plant in the Antarctic becomes operational. |
| 1963 | President Kennedy ratified the Limited Test Ban Treaty for the United States on October 7. |
| 1964 | President Johnson signed law permitting private ownership of certain nuclear materials. |