Question

Who discovered the presence of electrons in an atom ? Discuss detailed experiment connected with it. How is its mass and charge determined ?

Answer 1

J.J. Thomson discovered the presence of electrons in an atom in 1897.

The experiment performed by J.J. Thomson:

1. In this experiment a cylindrical hard glass tube closed at both ends was taken. It is called a discharge tube. Two metallic electrodes are fitted and connected to a high voltage battery. The negative terminal was called the cathode and the positive terminal was called the anode. The discharge tube is filled with air or gas.
2. Now, a current of high voltage was passed through the discharge tube under very low pressure nearly 0.01 atm, Electricity started flowing through the tube. As a result, light is emitted into the tube.
3. When the pressure of the gas is further reduced to 0.0001atm the emission of light in the tube stops. The walls of the discharge tube opposite the cathode started glowing with a greenish light called fluorescence.
4. This glow was due to the bombardment of the glass by certain rays emitted from the cathode. These rays moved from cathode to anode. These rays were named cathode rays.
5. These rays consist of negatively charged particles. Therefore, particles of cathode rays were named electrons.

Mass and charge of the electron were found by Millikan's oil drop experiment in which an e/m ration was used.