The road to the atomic bomb began in earnest in 1919, when New Zealander Ernest Rutherford reported on a series of experiments he had been conducting, which involved the bombardment of light element nuclei with energetic α (alpha) particles. Rutherford reported that nitrogen nuclei ejected what he suspected was "a hydrogen atom" (a proton). He concluded the nitrogen atom was "disintegrated" in the process, and he subsequently asked Patrick Blackett (a research fellow working under Rutherford) to study what precisely was happening. For the next four years Blackett used a cloud chamber to observe some 400,000 alpha particle tracks, which ultimately revealed that the nitrogen atom being bombarded had been transformed into an oxygen isotope in the process. Blackett discovered that the process was not one of disintegration but one of integration, and he published his discovery of the atomic transmutation of nitrogen into oxygen in 1925.
The final addition to the atomic "miniature solar system" first proposed by Niels Bohr came in 1932 when James Chadwick, Rutherford's colleague at Cambridge, identified the third and final basic particle of the atom: the neutron.
By the early 1930s, the atom was thought to consist of a positively charged nucleus, containing both protons and neutrons, circled by negatively charged electrons equal in number to the protons in the nucleus. The number of protons determined the element's atomic number. Hydrogen, with one proton, came first and uranium, with ninety-two protons, last on the periodic table.
This simple scheme became more complicated when chemists discovered that many elements existed at different weights even while displaying identical chemical properties. It was Chadwick's discovery of the neutron in 1932 that explained this mystery. Scientists found that the weight discrepancy between atoms of the same element resulted because they contained different numbers of neutrons. These different classes of atoms of the same element but with varying numbers of neutrons were designated isotopes. The three isotopes of uranium found in nature, for instance, all have ninety-two protons in their nuclei and ninety-two electrons in orbit. But uranium-238, which accounts for over ninety-nine percent of natural uranium, has 146 neutrons in its nucleus, compared with 143 neutrons in the rare uranium-235 (.7 percent of natural uranium) and 142 neutrons in uranium-234, which is found only in traces in the heavy metal. The slight difference in atomic weight between the uranium-235 and uranium-238 isotopes figured greatly in nuclear physics during the 1930s and 1940s.
