Discovery of electron

Production of cathode rays

In 1885, Sir William Crookes carried out a series of investigations into the behaviour of metals heated in a vacuum. The experiment of Crookes and others showed that a heated cathode produced a stream of radiation, which could cause gases at low pressure to glow and which, make other substances emit light too. The radiation emitted from the cathode was given the name 'Cathode rays'. By mid-nineties it was known that these rays could be deflected by a magnetic field and they carried a negative charge. Some scientists felt that these rays were waves and others were inclined to think they were particles.

Perrin tube

In 1897, J J Thomson showed that the stream of particles were indeed electrons. He conducted the famous discharge tube experiment by passing electricity at high voltage through a gas at low pressure.

A common discharge tube is a long glass tube having two metal plates, sealed at its two ends as electrodes. It has a side tube through which air can be pumped out by using a vacuum pump, so that experiments can be performed at low pressure.

Production of Cathode rays

When the pressure of air in the discharge tube is reduced to .001 mm of mercury and a high voltage is applied to the electrodes, the emission of light by air stops. But the phenomenon of fluorescence is observed in which the walls of the discharge tube at the end opposite to the cathode begin to glow with a greenish light. It is now deduced that some invisible rays were formed at the cathode, which on striking the glass tube emitted a green light. Since they are formed at the cathode they are known as cathode rays.

Properties (Experimental observations)

Travel in straight lines

When an opaque object like a metal cross is placed in the path of cathode rays in a discharge tube, a shadow of the metal cross is formed at the end opposite to the cathode.

Cathode rays cast shadows of the objects placed in their path

Produce mechanical effects

On placing a light paddle in the path of cathode rays in a discharge tube the blades of the paddle wheel rotate. This shows that cathode rays are a beam of particles having mass and possessing kinetic energy.

Cathode rays can rotate a high paddle wheel placed in their path

Are negatively charged

When an electric field is applied in the path of cathode rays, they are deflected towards the positive plate of the electric field, which shows cathode rays are made up of negatively charged particles.

Effect of electric field

Effect of electric field on cathode rays

• The nature of cathode rays does not depend on the nature of gas taken in the discharge tube or material of the cathode.

• The ratio of the charge to mass (e/m ratio) of cathode ray particles obtained from different gases was found to be exactly the same.

Conclusion

Since all gases form cathode rays, it means that all atoms contain electrons. The cathode ray particles being negatively charged an electron is negatively charged.

Characteristics of an electron

The mass of an electron is 1/1840 of the mass of a hydrogen atom. Since the mass of hydrogen atom is 1 amu., the relative mass of an electron is 1/1840 amu. The absolute mass is 9 x 10^-28 gram.

Charge of an electron

An electron is found to carry 1.6 x 10^-19 coulomb of negative charge. Since this is the smallest negative charge carried by any particle, it is taken as unit negative charge.