EXPLORING THE ATOM
(1919-1932)
Events: Atomic Discoveries,
1890s-1939

The road to the atomic bomb began in earnest in 1919 with the first artificial transmutation of an element.  The New Zealander Ernest Rutherford, working in the Cavendish Laboratory at Cambridge University in England, changed several atoms of nitrogen into oxygen.  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.  

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 type of particle accelerator (dubbed a "Cockroft-Walton machine") 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 at the University of California, Berkeley (right).