Physical Chemistry (3rd Edition) Heys
Page 116
Neutrons
For a long time it was thought that atoms consisted only of electrons and protons. However, in 1932 it was found that when beryllium was bombarded with gamma-rays particles were given off what had properties quite different from those of electrons and protons. They were not deflected at all by electric or magnetic fields, and therefore possessed no charge. The new particles were investigated by Chadwick, who showed that they had almost the same mass as a proton. As they were neutral particles, they were called neutrons. Later experiments have shown that neutrons represent the third of the fundamental particles which make up atoms.
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