Fundamental Particles

Fundamental Particles

In 1878, Sir William Crookes, while conducting an experiment using a discharge tube, found certain visible rays travelling between two metal electrodes. These rays are known as Crookes’ Rays or Cathode Rays. The discharge tube used in the experiment is now referred as Crookes tube or more popularly as Cathode Ray Tube (CRT).

Cathode Ray Tube is a long glass tube filled with gas and sealed at both the ends. It consists of two metal plates (which act as electrodes) connected with high voltage. The electrode which is connected to the negative terminal of the battery is called the cathode (negative electrode). The electrode connected to the positive terminal is called the anode (positive electrode). There is a side tube which is connected to a pump. The pump is used to lower the pressure inside the discharge tube.

Electricity, when passes through air, removes the electrons from the gaseous atoms and produces cations. This is called electrical discharge.

1. Discovery of Electrons

When a high electric voltage of 10,000 volts or more is applied to the electrode of a discharge tube containing air or any gas at atmospheric pressure, no electricity flows through the air. However, when the high voltage of 10,000 volts is applied to the electrodes of discharge tube containing air or any gas at a very low pressure of about 0.001 mm of mercury, a greenish glow is observed on the walls of the discharge tube behind anode. This observations clearly show some invisible ray coming from thecathode. Hence, these rays are called cathode rays. Later, they were named as electrons.

The fact that air is a poor conductor of electricity is ablessing in disguise for us. Imagine what would happen if air had been a good conductor of electricity. All of us would have got electrocuted, when a minor spark was produced by accident.

Properties of Cathode rays

• Cathode rays travel in straight line from cathode towards anode.

• Cathode rays are made up of material particles which have mass and kinetic energy.

• Cathode rays are deflected by both electric and magnetic fields. They are negatively charged particles.

• The nature of the cathode rays does not depend on the nature of the gas filled inside the tube or the cathode used.

In television tube cathode rays are deflected by magnetic fields. A beam of cathode rays is directed toward a coated screen on the front of thetube, where by varying the magnetic field generated by electromagnetic coils, the beam traces a luminescent image.

2. Discovery of Protons

The presence of positively charged particles in the atom has been precisely predicted by Goldstein based on the conception that the atom being electrically neutral in nature, should necessarily possess positively charged particles to balance the negatively charged electrons.

Goldstein repeated the cathode ray experiment by using a perforated cathode. On applying a high voltage under low pressure, he observed a faint red glow on the wall behind the cathode. Since these rays originated from the anode, they were called anode rays or canal rays or positive rays. Anode rays were found as a stream of positively charged particles.

When invisible radiation falls on materials like zinc sulphide, they emit a visible light (or glow). These materials are called fluorescent materials.

Properties of Anode rays

• Anode rays travel in straight lines.

• Anode rays are made up of material particles.

• Anode rays are deflected by electric and magnetic fields. Since, they are deflected towards the negatively charged plate, they consist of positively charged particles.

• The properties of anode rays depend upon the nature of the gas taken inside in the discharge tube.

• The mass of the particle is the same as the atomic mass of the gas taken inside the discharge tube.

When hydrogen gas wastaken in a discharge tube, the positively charged particles obtained from the hydrogen gas were called protons. Each of these protons are produced when one electron is removed from one hydrogen atom. Thus, a proton can be defined as an hydrogen ion (H+).

H  →  H⁺ + e^–

3. Discovery of Neutrons

At the time of J.J.Thomson, only two fundamental particles (proton and electron) were known. In the year 1932, James Chadwick discovered another fundamental particle, called neutron. But, the proper position of these particles in an atom was not clear till Rutherford described the structure of atom. You will study about Rutherford’s atom model in your higher classes.

Properties of Neutrons

• Neutron carries no charge. It is a neutral particle.

• It has mass equal to that of a proton, that is 1. 6 × 10^–24 grams.

Activity 1

Collect more information about the properties of fundamental particles and prepare a chart.