Discovery of Neutron:
From the previous discussion, we can see that the gold foil experiment gave a clear picture of the structure of an atom which consists of protons (nucleus) and same number of electrons outside of the nucleus.
Figure 6. Schematic diagram for the experiment that led to the discovery of neutrons by Chadwick.
94Be+42α⟶[136C]⟶[126C]+10n
But scientists soon realized that the atomic model offered by Rutherford is not complete. Various experiments showed that mass of the nucleus is approximately twice than the number of proton. What is the origin of this additional mass? Rutherford postulated the existence of some neutral particle having mass similar to proton but there was no direct experimental evidence.
Several theories and experimental observations eventually led the discovery of neutron. We can summarize some of the scientific observations behind the discovery of neutron.
· In 1930, W. Bothe and H. Becker found an electrically neutral radiation when they bombarded beryllium with alpha particle. They thought it was photons with high energy (gamma rays).
· In 1932, Irène and Frédéric Joliot-Curie showed that this ray can eject protons when it hits paraffin or H-containing compounds.
· The question arose that how mass less photon could eject protons which are 1836 times heavier than electrons. So the ejected rays in bombardment of beryllium with alpha particles cannot be photon.
· In 1932, James Chadwick performed the same experiment as Irène and Frédéric Joliot-Curie but he used many different target of bombardment besides paraffin. By analyzing the energies of different targets after bombardment he discovered the existence of a new particle which is charge less and has similar mass to proton. This particle is called neutron. Beryllium undergoes the following reaction when it is bombarded with alpha particle.
94Be+ 42α⟶[136C] ⟶ [126C]+ 10n
Here the symbol XZA is used where Z = atomic number and X = atomic mass of the element A. 
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