The Manhattan Project: Making the Atomic Bomb
Part I: Physics Background, 1919-1939
The Atomic Solar System
The road to the atomic bomb began in 1919 when the New Zealander Ernest Rutherford, working in the Cavendish Laboratory at Cambridge University in England, achieved the first artificial transmutation of an element when he changed several atoms of nitrogen into oxygen [editor's note: this research was conducted at Manchester University in England, this is a common error]. At the time of Rutherford's breakthrough, the atom was conceived as a miniature solar system, with extremely light negatively charged particles, called electrons, in orbit around the much heavier positively charged nucleus. In the process of changing nitrogen into oxygen, Rutherford detected a high-energy particle with a positive charge that proved to be a hydrogen nucleus. The proton, as this subatomic particle was named, joined the electron in the miniature solar system. Another addition came in 1932 when James Chadwick, Rutherford's colleague at Cambridge, identified a third particle, the neutron, so-named because it had no charge.
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, 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.
The year 1932 produced other notable events in atomic physics. The Englishman J. D. Cockroft and the Irishman E. T. S. Walton, working jointly at the Cavendish Laboratory, were the first to split the atom when they bombarded lithium with protons generated by a particle accelerator and changed the resulting lithium nucleus into two helium nuclei. Also in that year, Ernest O. Lawrence and his colleagues M. Stanley Livingston and Milton White successfully operated the first cyclotron on the Berkeley campus of the University of California.