Discovery of Electron
The electrons were first identified by cathode ray tube (or electric discharge tube) by JJ Thomson in 1887. Many other scientists like Faraday, Crooks and Goldstein studied the effects of passing electric current through a gas. As a result a sub-atomic particle electron with a negative charge was discovered.
Discharge Tube Experiment
• A gas discharge tube is a tube fitted with two metallic electrodes which act as cathode and anode
• The tube is filled with a gas, air or vapours of a substance at any desired pressure
• The electrodes are connected to a source of high voltage The tube is attached to a. vacuum pump
In the beginning, an electric current was passed through the gas in the discharge tube at ordinary pressure. The gas in the tube was not affected even at high potential of 5000 volts. Then the gas was discharged at a low pressure of 0.01 torr and given high voltage of 5000-10,000 volts
It was observed that the original glow disappeared, the gas became conductor, current started to flow and the gas started to emit light (example of discharge tube is a road neon sign). When the pressure is reduced further, emission of light by the gas ceases. Certain rays were given out from cathode and travel towards anode. The rays emitted were called cathode rays because they originated from cathode.
Properties of cathode Rays
Some systematic studies were made by certain scientists in order to investigate the properties of cathode rays. These properties are mentioned below:
Effect of electric and magnetic fields.Cathode rays are negatively charged particles. J Perrin (1895) showed that cathode rays are deflected in a magnetic field. J.J Thomson (1897) proved that these rays can be deflected towards anode showing that they are negatively charged. They produce a greenish fluorescence on striking the walls of the glass tube.
Straight line Motion
Hittorf (1869) proved that cathode rays cast a sharp shadow when an opaque object is placed in their path. This proves that they travel in straight line perpendicular to the surface of cathode.
They can drive a small paddle wheel placed in their path. This verifies that they are material particles and have certain momentum also.
X-Rays Production They can produce X-rays when they strike on an anode particularly with large atomic mass.
Heating effect They produce heat when they fall on a platinum foil and foil begins to glow.
Ionization They can ionize gases
Chemical Effect They can cause a chemical change in a material on which they fall
Penetration They are capable of penetration in metallic sheets like a aluminum or gold
J. J. Thomson determined e/m value of an electron. He concluded that all atoms contain electrons. The value of e/m is 1.7588 x 1011 coulombs kg-1
Whatever the gases and the vapours in the discharge tube, the cathode rays and the electrons are always the same.
This proves that electrons are fundamental particles of all atoms.