Name: Electrovalency
Uploaded: 2019-07-19
Duration: 1 min 51 s
Description: Do you know how chemical bonds are formed? They are formed in order to attain a stable electronic configuration. Let's watch the video to learn about the pro...

Question

How were cathode rays discovered by Thomson?  Describe with the help of an experiment.

Accepted Answer

The electrons were identified by J.J. Thomson in an experiment performed in a discharge tube. It is a long cylindrical tube sealed at both the ends and is fitted with two metal electrodes.

The electrode connected to the negative terminal of the battery is called cathode while the one connected to its positive terminal is anode.

The two electrodes are connected to a source of high voltage. The tube is connected to a vacuum pump to reduce the pressure inside the tube to any extent. The discharge tube was filled with air under study and the following observations were noted:

(i) A current of very high voltage was passed through the vacuum pump. A light was emitted inside the tube.

(ii) When emission of light was stopped, the wall of the discharge tube, opposite to the cathode, resulted in a greenish light, which was called fluorescence.

It was found that the fluorescence is due to the bombardment of the walls of the tube by the rays emitted from the cathode. These were known as cathode rays. Thomson repeated the experiment by replacing air with a number of gases and also changing the metal which constituted cathode. But he found that in all the experiments, cathode rays emitted were similar.

Mass number	Atomic number	Number of protons	Number of neutrons	Name of the element
$31$	$15$	$-$	$-$	$-$
$-$	$-$	$12$	$12$	$-$
$-$	$17$	$-$	$18$	$-$

How were cathode rays discovered by Thomson? Describe with the help of an experiment.

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