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Neutrons were discovered because the mass of an atom was not equal to the number of protons. Scientists therefore hypothesized that there was a neutral mass that increased the atoms overall mass.
This idea was originally proposed by Ernest Rutherford in 1920. In 1930 scientists bombarded beryllium with alpha particles. The particles produced had good penetrating power, which suggested a large mass. The particles was also not affected by a magnetic field. From these two experiments, what evidence led to the conclusion of the existence of neutrons?
Chadwick repeated these experiments and observed the energy of the neutrons. By oberving the particles' velocity, he concluded that the mass of a neutron was equal to that of a proton.
Applications:Neutrons can be used to start nuclear fission--an atom is bombarded with neutrons until the atom splits, releasing energy.