Since the atom as a whole is electrically neutral and the presence of negatively charged particles in it was established ,therefore, it was thought that some positively charged particles must also be present in the atom.
Goldstein in 1886 performed discharge tube experiment in which he took perforated cathode and a gas at low pressure was kept inside the tube, as before.
1)On passing high voltage between the electrodes ,it was found that some rays were coming from the side of the anode which passed through the holes in the cathode and produced green fluorescence on the opposite glass wall coated with zinc sulphide. These rays were called anode rays or canal rays or positive rays.
Properties of anode rays
1)They travel in straight line.
2)They are made up of material particles.
3)They are positively charged.
4)The value of e/m depend upon the nature of gas taken inside the discharge tube.
5)The value of the charge on the particles constituent the anode rays is also found to depend upon the nature of the gas taken inside the discharge tube.
6)The mass of the particle constituting the anode rays is also found to be different for different gases taken in discharge tube.
The charge on these particles is found to be same as that on the electron i.e. 1.6 × 10-19 coulombs per gram.
The ratio, charge/mass, for each of the particle is found to be 9.58 × 10-24 g.
These particles were termed as protons.
A proton may be defined as that fundamental particle which carries 1 unit positive charge and has a mass nearly equal to that of hydrogen atom.
Discovery of neutron
Chadwick in 1932 ,performed some scattering experiment in which he bombarded some light elements like beryllium and boron with fast moving Alpha particles. He found that some new particles were emitted which carried no charge ie. were neutral but had a mass nearly equal to that of proton. This particle was termed neutrons.
A neutron may be defined as that fundamental particles which carries no charge but has a mass nearly equal to that of hydrogen atom or proton.