Question

With reference to the discovery of the structure of an atom, explain in brief – William Crookes experiment for the discovery of cathode rays, followed by – J.J. Thomsons experiment pertaining to the constituents of the cathode rays. State which sub-atomic particle was discovered from his experiment.

Answer 1

Electrons were discovered in 1897 by J.J. Thomson when he was studying the properties of cathode rays.

Earlier, William Crooks, another British scientist, had performed an experiment to study the phenomenon of electric discharge through gases. He observed that when an electric current of high voltage was passed through a discharge tube (a glass tube sealed at both ends with metal plates) containing a gas at very low pressure (0·01 mm of mercury), rays were emitted from the negative terminal called cathode. He called these rays 'cathode rays'.

J.J. Thomson studied the characteristics and the constituents of the cathode rays and concluded that: Cathode rays consist of negatively charged particles (now called electrons), present in atoms of all the elements.

J.J. Thomson's Experiment: An electric field was applied in the path of cathode rays in the discharge tube. It was observed that cathode rays were deflected towards the positive plate of the electric field. This showed that cathode rays were negatively charged.

When a magnetic field was applied in the path of cathode rays, they were again deflected in a direction in which moving negative charge would be deflected.

This proved that cathode rays contained negatively charged particles called electrons.