Discovery of Electron, Proton & Neutron

Discovery of Electron:

In 1897, J. J. Thompson produced experimental evidence for the existence of sub-particles known as electrons in the atom of every element (using the Cathode Ray Experiment).

He showed that when a high electrical potential was applied at very low pressure (about 0.001 mm Hg) on a glass tube using a cathode ray tube, the rays were observed to be emitted from the negative electrodeElectrodes are conductors, in the form of wires, rods or plates, through which current enters and leaves the electrolyte. When the current leaves the electrodes it is known as the cathode... More (cathode) to the positive electrode (anodeAn anode is an electrode of a polarized electrical device through which conventional current enters the device. It is the positive part of electrolytes where oxidation takes place. More).

The rays were also found to be deflected by magnetic and electric fields. Since they emanated from the cathode to the anode, they were called cathode rays. It was then concluded that they must be of opposite charge for them to deflect towards the positive electrode, hence, they must be negatively charged particle electrons.

Cathode Rays:

Cathode rays are streams of negatively charged particles moving in straight lines with high velocities from the Cathode to the Anode.

Properties of Cathode Rays:

1. The rays are emitted at right angles to the electrodes.

2. They travel in straight lines with a high velocity.

3. The rays are deflected by electric and magnetic fields.

4. The rays flow from the cathode to the anode and are deflected towards the positive plate of an electric field.

5. The rays cast a shadow of an object placed on their path; it shows that the rays have low penetrating power.

6. They heat a metal foil placed between the electrodes.

7. The rays are capable of producing mechanical motion.

Discovery of Proton:

Since atoms are electrically neutral, they should exist as positively charged components to balance the negative charge of the electrons.

Thompson repeated his earlier experiment but used a discharged tube with a perforated cathode plate and then connected it to a current.

It was observed that some rays passed through the perforated cathode moving in the opposite direction to that of cathode rays. These rays were deflected by electric and magnetic fields in the opposite manner to electrons showing that they are positively charged particles called protons.

Discovery of Neutron:

In 1932, James Chadwick bombarded a beryllium atom with fast-moving alpha particles. He discovered the existence of a new particle which carried no charge i.e it is neutral and has a mass almost equal to that of a proton. The new particle is called a neutron.

Millikan Oil-drop Experiment:

In 1909, R.A. Millikan used the oil-drop technique to determine the value of the charge on an electron. The value which was found to be 1.60 × 10^-19 coulombs was taken to be the value of the charge on an electron.

Rutherford’s Atom: 

Lord Rutherford, in 1911, used positively charged particles called Alpha particles to bombard a thin-gold foil. He found that most of the alpha particles passed through the foil, while a few of them were deflected back.

Rutherford explained his findings as follows:

a. Since most of the alpha particles passed through without being deflected, the atoms in the goldfield must contain a large space in which negative particles move.

b. Since very few of the alpha particles were deflected, the deflection might have been caused by a small region of the atom which consists of positive particles. The positive particles might have collided with the alpha particles leading to the deflection.

c. Among the particles deflected, a few were deflected at large angles. This shows that the positive particles in the atom must be quite heavy.

Rutherford concluded by proposing a nucleus theory of an atom. According to his theory, the atom consists of a small nucleus which contains protons and neutrons while electrons revolve around the nucleus.

Henry Moseley in 1914 suggested that the number of protons in the nucleus is a fundamental characteristic of an atom, based on the results of his x-ray experiment on the elements.

Moseley called the number of protons the atomic number and the atomic number is equal to the number of electrons.

Further experiments showed that the nucleus contained two particles of approximately equal masses. These are protons which have a positive charge and neutrons which have no charge. Protons and Neutrons have a mass of one and the electron is negatively charged.