Search for the missing particle
Although Rutherford's model was enormously successful at explaining the scattering of alpha particles, there was a problem when the experimental data were used to calculate the mass of the nucleus. From the results of the scattering experiments, it was possible to calculate the charge on the nucleus. However the mass of these protons was only about half the overall mass of the nucleus. In 1920, William Draper Harkins an American physicist suggested that the missing mass could be accounted for if the nucleus contained other particles with mass similar to that of a proton but no charge. He named this particle neutron. James Chadwick finally discovered it in 1932. He bombarded the element beryllium with alpha-particles. He observed the emission of a radiation with the following properties:
- The radiation was highly penetrating
- The radiation remained unaffected in an electric or magnetic field i.e., the radiation was neutral
- The particles constituting the radiation had the same mass as the proton. Thus the relative mass of such a particle = 1 amu and the absolute mass = 1.6 x 10-24g. Because of their electrical neutrality, these particles were called neutrons
We are now in a position of explain why the atomic mass of carbon is 12. It is now known that the carbon atom contains 6 protons and 6 neutrons each having a mass of 1 amu.
Atomic mass of carbon = Mass of 6 protons + Mass of 6 neutrons6 x 1 + 6 x 1 = 12