Ernest Rutherford (1871-1937) had determined that the charge on the alpha particle, actually the helium nucleus, was two proton charges, but its mass was four proton masses. Dmitri Mendeleev (1834-1907), in building the periodic table, had arranged the elements in order of their mass and had arbitrarily assigned them sequential numbers that he called the atomic number. The English physicist Henry Moseley (1887-1915) measured the wavelengths of X rays emitted when metals were struck by other X rays. He was able to provide a physical basis for the atomic number and, in doing so, greatly strengthened the case for Rutherford’s nuclear model.

But, as is the case for helium, if the atomic number, and thus the nuclear charge, was about half the atomic mass number, what made up the additional mass of the nucleus? The first proposal was that the missing particle was a combination of a proton and an electron, which would have the correct mass and charge. But the Heisenberg uncertainty principle showed that if an electron were confined to the size of a proton its energy would be larger than ever observed. In addition, by the late 1920s the angular momentum of the nitrogen nucleus, with charge 7 and mass 14 had been measured. The result could not be obtained from a combination of 14 protons and 7 electrons.

In 1931 two German physicists found that when energetic alpha particles struck light elements a very penetrating, neutral radiation was produced. The next year Marie Curie’s daughter, Irene Joliot-Curie (1897-1956), and her husband, Frederick, found that if this radiation struck paraffin protons were ejected, suggesting that the radiation was actually a neutral particle with a mass near that of a proton. The next year James Chadwick (1891-1974), working in Manchester, England, experimentally confirmed the suggestion. The particle was named a neutron, combining “neutral” with the ending of the word proton. He was awarded the Nobel Prize for his discovery in 1935.

The neutron has a mass slightly larger than that of the proton. While neutrons are stable in non-radioactive nuclei, if they are free from the nucleus they decay with a half-life of about 10 minutes. Neutrons are used extensively in creating nuclear reactions and are necessary for nuclear fission, which will be discussed later in this chapter.