Discovery of protons - Study of Anode rays, Discovery of Neutron

 

Discovery of proton - Study of anode rays

Goldstein in 1886, performed discharge tube experiments in which he took perforated cathode and a gas at low pressure was kept inside the tube .

                                        

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 ( because they passed through the canals, i.e., holes in the cathodes ) or positive rays ( because they were found to carry positive charge ).

Origin of Anode rays

These rays are believed to be produced as a result of the knock out of the electrons from the gaseous atoms by the bombardment of high speed electrons of the cathode rays on them. Thus anode rays are not emitted from the anode but are produced in the space between the anode and the cathode.

Properties of Anode rays

(i) They travel in straight lines. However, their speed is much less than that of the cathode rays.
(ii)They are made up of material particles.
(iii) They are positively charged , as indicated by the direction of deflection of these rays in the electric and magnetic fields. That is why they are called 'positive rays'.
(iv) Unlike cathode rays , the ratio, charge/mass (i.e. , e/m) is found to be different for the particles constituting anode rays when different gases are taken inside the discharge tube. In other words, the value of e/m depends upon the nature of the gas taken inside the discharge tube.
(v) The value of the charge (e) on the particles constituting the anode rays is also found to depend upon the nature of the gas taken inside the discharge tube.
(vi) The mass (m) of the particles constituting the anode rays is also found to be different for different gases taken in the discharge tube.

Proton(H⁺)

A proton is the fundamental particle which carries one unit positive charge(1.6 × 10^-19C) and has a mass nearly equal to that of hydrogen atom (1.67 × 10^-24 g).

Discovery of Neutron

Neutron was discovered by Chadwick,in 1932, by bombarding Be or B atoms with fast moving α -particles (He⁺⁺nucleus). Neutral particles with mass nearly equal to that of proton were emitted. These were called neutrons.
Thus, neutron is defined as that fundamental particle which carries no charge but has mass nearly equal to that of H-atom.

Charge and mass of fundamental subatomic particles

Particle	Discoverer	Charge	Mass in kg	Mass in u
Electron	J. J. Thomson	-1	9.10939 × 10-31	0.000548596
Proton	Goldstein	+1	1.67262 × 10-27	1.00727663
Neutron	James Chadwick	0	1.67493 × 10-27	1.0086654

One unit charge = 1.60210 ×10^-19 coulomb
One u (or 1amu ) = 1/12th the mass of C-12 atom