James Chadwick (1891-1974)

Current Atomic Model

1. Chadwick in collaboration with Rutherford realized that the atomic mass of most elements was about double the number of protons. For example, helium has an atomic mass of about 4 and only 2 protons. Electrons have minimal relative mass, so they postulated that a neutral particle may exist.
2. Irene (Marie Curie's daughter) and Frederic Joliot-Curie were conducting many radiation experiments during this time. However, they were not searching for neutrons. Chadwick realized that some of their experiments might be used to find the elusive neutron, so he repeated their experiments with this goal in mind.
3. He measured the mass and velocity of atoms knocked out of material placed near a radioactive source. Through these measurements he was able to determine the mass and energy of the neutral radiation that knocked them free, thus measuring the mass of individual neutrons, and providing an explanation of why atoms were being ejected from a source of radiation previously undiscovered.
4. In 1932, he found definitive evidence for the existence of a neutral particle that had roughly the same mass as a proton. He discovered the neutron.