Cathode Rays and Discovery of Electron

This post includes the topics of Class 9 Chapter 4 Structure of Atom, i.e., Cathode rays and the JJ Thomson model and the discovery of electron.

Atoms are too small to be seen even with the most powerful microscope and, too light to be weighed even on the most sensitive balance. In this chapter, we will discuss only the most important contributions which helped us in sharing our ideas about the structure of atoms.

Electric nature of matter: Discharge tube experiment

What is meant by electrical discharge through gases?

Electrical discharge through gases studied by using a specially designed glass tube commonly called a discharge tube. It consists of a cylindrical glass tube having a side tube and two metallic electrodes, present at each end. These electrodes can be connected to the respective terminals of a high tension power supply. Air from inside the tube can be pumped out by connecting the site tube to a vacuum pump and a desired pressure can be maintained inside the tube.

It was found that gases do not conduct electricity even when an electric potential of about 10000 V is applied. But it was discovered by William Crookes that gas could conduct electricity at low pressure. When the pressure inside the discharge tube was reduced gradually, the following observations were made:

• At 10^-2 atm pressure: At about 10^-2 atm pressure, a glow surrounding the cathode, negative electrode, leaves the electrode surface, and little space is left between it and the electrode. This is called Crooke’s dark space. At this stage, electric current begins to flow from one electrode to the other electrode.
• At 10^-3 atm pressure: At sufficiently low pressure, this glow fills the whole tube. The colour of the glow depend upon the nature of gas in the tube, and on the colour of the gas used for making the discharge tube.
• At 10^-5 atm pressure: When the pressure is lowered to about 10^-5 atm, light emission by the residual air in the discharge tube stops. But the walls of the discharge tube opposite the cathode start glowing with light greenish light. At this stage, a stream cold cathode rays are emitted from the cathode.

Thus, we see that when electric discharge is passed through gases at very low pressures, cathode rays are produced.

Cathode Rays

How are cathode rays produced?

When a high very high electric potential, ~10000 V, is applied across a gas taken in a discharge tube at very low pressure, about 10^-5 atm, some radiations are emitted from the cathode. These radiations are called cathode rays. The emission of these rays in a discharge tube is shown in figure.

At this stage, the glass (walls) off the discharge tube, opposite to cathode, starts glowing with a faint greenish light. This is due to a phenomenon called fluorescence. This fluorescence of walls is due to the bombardments of glass by rays emitted from the cathode. Therefore, these rays are called cathode rays.

What are the properties of cathode rays?

Cathode ray show the following properties:

• Cathode ray travel in straight lines. That is why, cathode rays cast a shadow of any solid object placed in their path. 

The path cathode ray travel is not affected by the position of the anode. 

• Cathode ray consists of matter particles and possess energy by the virtue of their mass and velocity. Cathode ray sets a paddle wheel into motion when it is placed in the path of these rays. This is due to the impact of the particles of the cathode rays on the blades of the paddle wheel.

• Cathode ray consists of negatively charged particles. When the cathode rays are subjected to an electric field, these get deflected towards the positively charged plate (anode). We know that a positively charged body would attract only a negatively charged body, therefore, the particles of cathode rays carry negative charge.

• Cathode ray also get deflected when these are subjected to a strong magnetic field.
• Cathode ray heat the object on which they fall. The cathode ray particles possess kinetic energy. When these particles strike an object, a part of the kinetic energy is transferred to the object. This causes a rise in the temperature of the object.
• Cathode ray cause green fluorescence and glass surface, i.e., the glass surface on which the cathode rays strike show a green coloured shine.
• Cathode ray can penetrate through thin metallic sheets.
• Cathode ray ionises the gases through which they travel.
• When cathode rays fall on certain metal such as copper, X-rays are produced. The X-rays are not deflected by electrical or magnetic fields. X-rays pass through opaque materials such as black paper, but are stopped by solid objects such as bones.
• Cathode rays travel with the high speed nearly equal to that of a speed of light.

J J Thomson’s experiment and the discovery of electron

Properties of the cathode rays showed that these consist of streams of negatively charged particles having very small mass.  J J Thomson called these particles corpuscles of negative electricity. Later, G J Stoney called these particles electrons.

Is electron an essential constituent of all matter?

In 1897, an English physicist J J Thomson, studied the effect of an electric field on cathode rays. The discharge tube used by Sir J J Thomson consisted of the following parts:

• a cathode
• a cylindrical metal disc having a fine hole at its center, acts as anode – this accelerates the particle of the cathode rays
• another metal disc having a fine hole at its center in line with the hole in the anode
• the cathode rays (consisting of electrons) pass through these holes and strike the fluorescent screen at point E’
• two flat plates P1 and P2, which can be connected to a source of high voltage
• an electromagnet generating field opposite to the field generated by the plates P1 and P2.

In the absence of any electrical or magnetic field, the cathode rays strike the fluorescent screen at point E, and can be seen as a bright spot there.

When a high electric field is applied across the plates P1 and P2, the spot on the screen moves towards the positively charged plate.

The bending of cathode rays towards the positive plate showed that the cathode rays constituted of negatively charged particles. The deflection suffered by the beam of cathode rays in Thomson’s experiment depends upon the strength of the electric field applied across electrodes.  

The charge to mass (e/m) ratio of an electron

J J Thomson studied the combined effect of electric and magnetic fields on the cathode rays. The two fields were applied in such a way that the deflection due to electric field is cancelled out by the deflection due to the magnetic field, so that the net deflection was zero.

From the strengths of the electric and magnetic fields required to balance the deflections, the ratio of charge to mass of the particles in the cathode rays was found out. It was found that the ratio of charge to mass of the particles in cathode rays is the same, and does not depend upon the nature of the gases used in the discharge tube. The charge to mass (e/m) value for the particles in the cathode rays was found to have a constant value of 1.76*10¹¹ C/Kg. From this experiment, Thomson concluded that all atoms contain the same type of negative particles called electrons.

Thus, it becomes clear that same particle is present in cathode rays even when different gases and electrodes of different materials used in discharge tube experiment. These negatively charged particles were given the name electron. So, electrons must be present in all matter, i.e., electrons are the essential constituent of matter.

The charge and mass of an electron

In 1909, an American physicist, Robert A. Millikan, accurately determined the charge on an electron by performing his well-known oil drop experiment. He is studied the motion of charged oil-drops in the space between two charged metal plates.

From the experiment, he found that charge on oil-drops was always a multiple of an elementary charge of 1.6*10-19 C. From these results and those obtained from studies of cathode rays, it was found out that the charge on the cathode ray particle was one unit of element charge. This quantity of electric charge was called electronic charge, e.

Thus,

Charge on an electron, e = 1.6*10^-19 C

e/m = 1.76*10¹¹ C/Kg

So, Mass of electron, m = 9.1*10^-31 Kg

What are the characteristics of an electron?

Electron is negatively charged particle having very small mass.

• Mass of an electron: The mass of an electron is about 1/1840 times that of a hydrogen atom. Its absolute mass is, m = 9.108*10^-31 Kg = 9.108*10^-28 g

It is very light particle, and therefore, it makes very little contribution to the total mass of the atom in which it is contained.

• Charge on an electron: An electron possesses one unit negative charge. It has been found to be the smallest negative charge that any particle can carry.

Charge on an electron, e = 1.602*10^-19 C

Read More: Atoms and Molecules

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