Class 9

9.2.2 Discovery of Electrons

This is the second lecture from Chapter 2: ‘Atomic Structure’ of the new book for Class 9 Chemistry (Punjab Board – PCTB). It covers the discovery of electrons through discharge tube experiments. The lecture includes a multiple-choice quiz, short-answer questions, and detailed long-answer notes.

MCQs Based Quiz

9.2.2 Discovery of Electrons

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1. The deflection of cathode rays towards positive charge indicates that:

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2. Cathode rays were negatively charged particles that are present in:

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3. Why does the glass behind the positive electrode begin to glow?

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4. When a very high voltage is applied to a gas at very low pressure,

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5. Cathode rays were later named as:

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6. Who calculated the mass-to-charge ratio of cathode rays?

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7. What effect does the magnetic field have on the cathode rays?

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8. When cathode rays pass through oppositely charged plates, they will

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9. Who studied the cathode rays that led to the discovery of electrons?

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10. The role of vacuum pump in discharge tube is to:

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Short Questions

Q1. What is a discharge tube, and what are its main components?

A discharge tube is a hard glass tube that has three main components:

  1. A vacuum pump to decrease the pressure of gas inside the tube
  2. A very high voltage power source
  3. Two electrodes at both ends of the tube.

Q2. How can you produce cathode rays in a discharge tube?

  1. The gas pressure is reduced using a vacuum pump.
  2. A high voltage power source is connected to the electrodes.
  3. A glow appears behind the positive electrode (anode), indicating the formation of cathode rays originating from the cathode.

Q3. Define cathode rays.

Cathode rays are negatively charged particles (electrons) that originate from the cathode when a very high voltage is applied across a discharge tube at low pressure.

Q4. What were two important observations J.J. Thomson made in the discharge tube experiment?

  1. The cathode rays were deflected toward the positively charged plate, indicating that they carry a negative charge.
  2. Their path was also deflected by a magnetic field

Q5. Write two properties of cathode rays.

  1. They are deflected towards the positively charged plate.
  2. They are also deflected by a magnetic field.

Descriptive Question

Q1. Write a comprehensive note on the discovery of electrons.

The discovery of electrons is primarily attributed to the discharge tube experiments.

Construction of a Discharge Tube:

A discharge tube is a hard glass tube that has three main components:

  1. A vacuum pump to decrease the pressure of gas inside the tube
  2. A very high voltage power source
  3. Two electrodes at both ends of the tube.
Cathode ray experiment apparatus

Production of Cathode Rays:

When a very high voltage is applied across the electrodes at very low gas pressure, a glow appears on the glass behind the positive electrode. This glow is caused by rays emitted from the cathode, known as cathode rays.

Observations of J. J. Thomson:

In 1897, a British physicist Joseph John Thomson conducted several experiments and made the following observations:

  1. When cathode rays were passed through oppositely charged electric plates, they bent towards the positive plate. This showed that the rays carry negative charge.
  2. A magnetic field also deflected the rays, further confirming their charged nature.
  3. Thomson measured the mass-to-charge ratio of these rays and concluded that they consisted of negatively charged particles.
  4. He identified these negatively charged particles as a new type of sub-atomic particle, later named electrons.

This discovery marked the first identification of a particle smaller than the atom and laid the foundation for modern atomic theory.