Class 11 Chemistry Structure of Atom Discovery Of Electron


The Electron was discovered by J.J Thomson by conducting a Cathode ray tube experiment.

For the experiment he used Crooke’s tube, which was 60cm long glass tube and had a small tube attached. To this small tube vacuum pump was attached, it also had two metal plates which were connected to battery by wires.


The tube contained gas at atmospheric pressure. when current at high voltage (10,000volts) was passed following observations were made:

  1. When current was passed through a gas at 1 atmospheric pressure and at a very high voltage, nothing happened. That is no visible effect was seen inside the tube.
  2. Then further the pressure of gas was reduced by pumping the air out, with the help of vacuum pump. The pressure was reduced to 10-2atm, then on passing current it was seen that whole tube started glowing green.
  3. Then further the pressure was reduced to 10-4It was seen the whole glow vanished, but it was seen that at the end of the tube (anode side) there was a faint green glow observed.

To confirm the faint glow anode was made perforated, and a zinc sulphide screen (fluorescent material) was placed behind it.


When current was passed under same conditions it also started   glowing green. This confirmed that under those conditions some rays were emitted through cathode, and were travelling towards anode. Those rays were called as cathode rays and found to consist of negatively charged particles called electron.


Properties of cathode rays

  1. They are found to travel in straight line. This property was concluded by performing an activity where the object was placed in their path. When they strike it ,they casted the shadow of the object as shown below:


  1. Cathode rays are formed of material particles. This property was concluded by performing an activity where in their path a paddle was mounted on an axle.When they strike it ,they rotated the paddle .



  1. They are negatively charged.This property was concluded by performing an activity where electric and magnetic field was applied, they deflected towards positive plate and in presence of magnetic field they deflected towards north pole.



  1. When they strike some metal surface, they heated it. So, it was concluded they can also produce heating effect.
  2. They cause ionization of gas through which they passed.
  3. They produce x-rays when strike against the hard metal like tungsten etc.
  4. They can make florescent material glow, when strikes some fluorescent material.
  5. They affect the photographic plate.
  6. They have penetrating effect.

From the above properties and experiment it was concluded that cathode rays are made up of negatively charged particles called electron.

Further experiments were carried out in order to know the charge and mass of electron. It was found to be:

  • Charge :1.6 x 10-19C
  • Mass 9.1 x 10-31kg

We can define electron as:

  • A fundamental particle that is negatively charge of magnitude 1.6 x 10-19C and mass equal to 9.1 x 10-31

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