Question

Explain in brief the experimental proof which led to the discovery of (1)electrons (2)protons(3) atomic nucleus (4) neutrons

For 5 marks

Answer 1

ELECTRON-
J. J. Thomson constructed a glass tube which was partially evacuated i.e. much of the air was pumped out of the tube. Then he applied a high electrical voltage between two electrodes at either end of the tube. He detected that a stream of particle (ray) was coming out from the negatively charged electrode (cathode) to positively charged electrode (anode). This ray is called cathode ray and the whole construction is called cathode ray tube.

PROTON-
Gold Foil Experiment
In his gold foil experiment, Rutherford bombarded a beam of alpha particles on an ultrathin gold foil and then detected the scattered alpha particles in zinc sulfide (ZnS) screen.

Discovery of Neutron-
Early in the century, Ernest Rutherford developed a crude model for the atom, based on the gold foil experiment of Hans Geiger and Ernest Marsden. In this model, atoms had their mass and positive electric charge concentrated in a very small nucleus.

NEUTRON -
In the year 1932, James Chadwick conducted bombarding beryllium with alpha particles but he used different bombardment targets other than paraffin. After research and analysis of the various targets, he discovered the presence of a new particle. This new particle has the mass similar to that of the proton but no charge. He named these particles neutrons.