After Rutherford discovered the proton in 1919, it seemed that in addition to the proton, there was another type of particle within the nucleus.
There was a discrepancy between the atomic number, or the number of protons in the nucleus which is equivalent to the positive charge of the atom, and the atomic mass, the average mass of the atom. For example, helium atoms had atomic masses of 4, but a positive charge/atomic number of 2. Since electrons have little to no mass, there had to be something else that was adding to the mass of the atom, but not the charge.
In 1932, Chadwick repeated experiments by Frederic and Irene Joliot-Curie that were originally used for tracking particle radiation, but instead focused on looking for a neutral particle. He found that there were neutrons that had 0.1% more mass than a proton's. Chadwick, however, believed that a neutron was a proton-electron pairing (which would negate the charge and cause it to have no charge), which was remediated when Werner Heisenberg proved that a neutron was its own unique particle.
When combined with Heisenberg's other discovery that an electron did not have a precise location, it formed the "electron cloud model," which proposed that there was a nucleus made up of protons and neutrons in the center of a cloud of fasting moving electrons that move about it.
There was a discrepancy between the atomic number, or the number of protons in the nucleus which is equivalent to the positive charge of the atom, and the atomic mass, the average mass of the atom. For example, helium atoms had atomic masses of 4, but a positive charge/atomic number of 2. Since electrons have little to no mass, there had to be something else that was adding to the mass of the atom, but not the charge.
In 1932, Chadwick repeated experiments by Frederic and Irene Joliot-Curie that were originally used for tracking particle radiation, but instead focused on looking for a neutral particle. He found that there were neutrons that had 0.1% more mass than a proton's. Chadwick, however, believed that a neutron was a proton-electron pairing (which would negate the charge and cause it to have no charge), which was remediated when Werner Heisenberg proved that a neutron was its own unique particle.
When combined with Heisenberg's other discovery that an electron did not have a precise location, it formed the "electron cloud model," which proposed that there was a nucleus made up of protons and neutrons in the center of a cloud of fasting moving electrons that move about it.