Discovery of electron

• Electron was discovered in cathode ray experiment.
• The name electron was proposed by Stoney for the fundamental unit of electricity.
• A current of high voltage (1000 volts) was passed through the discharge tube under normal pressure of one atmosphere. No electricity could flow through air as it is a very poor conductor of electricity.
• The pressure inside the discharge tube was slowly reduced by the vacuum pump. On reducing it to about 1mm of mercury and passing electric discharge again, electricity started flowing through the tube. As a result light was emitted inside the tube.
• The emission of light stopped when the pressure inside the tube was further reduced to about 0.001 mm of mercury. The walls of the discharge tube opposite to the cathode started glowing with the greenish light called fluorescence.
• The fluorescence is due to the bombardment of the walls of the tube by the rays emitted by the cathode. These were known as cathode rays.

• Thomson repeated the experiment by replacing air with number of gases and also changing the metal that constituted the cathode.
• He found that in all experiments cathode rays emitted were similar.

Characteristics of cathode rays:

• Cathode rays are emitted from the cathode.
• Cathode rays always travel in straight line.
• Cathode rays consist of negatively charged particles known as electrons.
• When cathode rays are made to strike on metal it gets heated.
• Cathode rays can pass through a very thin foil of a metal.
• Cathode rays form diffused spots on photographic plate or film. This is known as fogging.
• When cathode rays strike the surface of metals like tungsten, copper, molybdenum etc. they produce X-rays.
• Cathode rays are deflected by electric and magnetic field.
• The particles constituting cathode rays were named as electrons by G. J. Stoney and it means particles or atoms of electricity.
• The value of charge to mass ratio (e/m) also known as specific charge of an electron was determined by J. J. Thomson.
• The specific charge of an electron was found to be independent of the nature of gas and electrode used which suggests that electrons are present in all the atoms.

Characteristics of an Electron:

• The two important characteristics of an electron are its mass and charge.

1. Mass of an Electron:

• The actual mass of an electron is 9.11 × 10^–28 g. It was calculated by calculated by J. J. Thomson.
• The mass of an electron is about of the mass of hydrogen atom.
• Since the mass of a hydrogen atom is 1 u, we can say that the relative mass of an electron is the mass of an electron is so small that it is considered to be negligible.
• This is why the mass of electrons is ignored while calculating the atomic mass of an element.

2. Charge on an Electron:

• The charge on an electron is –1.602 × 10^–19 coulomb. It was determined by Mulliken in his oil drop experiment.
• The relative charge of an electron is –1.