James Chadwick found the third elementary particle of the atom; the neutron. In 1919 Ernest Rutherford discovered the proton, a positively charged particle contained in the nucleus of the atom. Many were convinced that the proton was not the only particle in the nucleus. Most of the atoms mass are in the nucleus, but if the nucleus was made up of only protons, the number of protons the atom contained did not make up for all of the atoms atomic mass. For example, a helium atom has an atomic mass of 4, but the atomic number (number of protons in the nucleus) is only 2. Electrons, the negatively charged particles that orbit the nucleus, have a very small mass, so the number of protons and electrons put together still did not make up for the atomic mass. One explanation, made by Ernest Rutherford, suggested that there was another particle inside the nucleus that had no charge. He called it the neutron, which he imagined as an electron and a proton paired up in the nucleus.
His explanation was not completely accurate, but he was on the right track. Chadwick explored further into the subject to find out what the unidentified mass was made of. He tried over and over again to find the neutron, and as much as he failed, he never gave up; because he was so sure it was there.
European physicists, Walter Bothe and Herbert Becker shot alpha rays at beryllium. The beryllium emitted a neutral radiation that could penetrate 200 millimeters of lead. This was odd, because it takes less than one millimeter of lead to stop a proton. Herbert and Bother thought the neutral radiation were high energy gamma rays. Chadwick was intrigued by the experiments of Federic and Irene Joliot-Curry. They used a different technique of tracking particle radiation by putting a block of paraffin wax in front of the rays coming from the beryllium, thought as to be gamma rays.
They observed high-speed protons coming out from the paraffin. Gamma rays eject electrons from metals; they thought the same thing was happening to the protons in the paraffin.
The beryllium rays were not gamma rays because the rays coming from the beryllium had far more energy. Chadwick recognized the beryllium rays as neutrons. Because of neutrons are neutral, they can penetrate thick layers of different materials because there movement is not disturbed by the positively or negatively charged particles in the penetrated material. Chadwick carried out his own experiments and confirmed that he had finally found the neutron.
Chadwick bombarded boron with alpha particles which emitted neutral rays, like beryllium, and placed a hydrogen target in the path of the rays. When the rays hit the hydrogen target, protons flew out. He measured the velocity of the protons. Using the laws of conservation of momentum energy he was able to calculate the mass of the neutral particle. From the velocity of the protons flying out, he could calculate the mass of the projectile, in this case, the mass of the neutrons. The mass of the neutral particle is 1.0067.