All of the exciting developments describing the electron orbiting around the nucleus did not mean that the nucleus was forgotten. The three types of radiation—a, p, and у—had been discovered as particles that were emitted from the nucleus of radioactive elements. But there were big problems with the nucleus. Rutherford had shown that the nucleus was a compact center of the atom filled with protons. Since protons have a positive charge and repel each other more strongly as they come close together, something must be holding them together, some new force different from electromagnetism. Not only that, but the masses of nuclei other than hydrogen were at least twice as large as the mass of the protons in the nucleus. For example, the a particles that Rutherford had shown to be helium nuclei had two protons but a mass equal to four pro­tons. Rutherford, in fact, proposed in a lecture in 1920 that there must be a neutral particle in the nucleus composed of a proton and an electron with no net charge to account for the extra mass—an idea that was wrong in the details but right in principle (10).

And new questions were being raised about the radiation coming from radio­active elements. Marie and Pierre Curie’s daughter, Irene Curie, and her husband, Frederic Joliot, were continuing the work of Marie Curie at the Radium Institute in Paris. They were studying the element beryllium by bombarding it with a parti­cles from polonium and found that the beryllium emitted radiation they thought was у rays. James Chadwick, a colleague and former student of Rutherford, did not believe that they were у rays. In a 10-day burst of frenetic activity in 1932, he bombarded different elements with the radiation coming out of beryllium and found that protons were knocked out of the nucleus of the different elements. In fact, he found that the energies of the protons exceeded the energies of the sup­posed у rays. But if the beryllium radiation was actually neutral particles with a mass nearly identical to protons, then the results were simple to explain. The effect is similar to a pool cue ball hitting a group of pool balls and knocking one of them out. Chadwick named the particle a neutron and changed the understanding of the nucleus (10). The new model of the nucleus consisted of protons and neu­trons of nearly identical mass that must have a force holding them together, later identified as the strong nuclear force. Because the strong force does not depend on charge, it acts equally on neutrons and protons, which are often referred to as nucleons.