Class 9 Science Exploration Solutions English Medium

NCERT Solutions Class 9 Science Exploration English Medium

Journey Inside the Atom

Topic: Thomsons Atomic Model - Discovery of Electrons

Chapter 8. Journey Inside the Atom

This section explains how the electron, the first subatomic particle, was discovered. It describes the cathode ray experiment conducted by J. J. Thomson, the properties of electrons, and Thomson's atomic model. These discoveries proved that atoms are not indivisible and contain smaller particles. 

Discovery of the Electron

Scientists performed several experiments to understand the internal structure of atoms. One of the most important discoveries was made by J. J. Thomson, who identified the electron through experiments using a cathode ray discharge tube.

Cathode Ray Discharge Tube

A cathode ray discharge tube is a glass tube containing gas at very low pressure with two metal electrodes connected to a high-voltage source. When electricity passes through the tube, invisible rays travel from the cathode towards the anode.

Cathode Ray Experiment

Experimental Setup

• A glass discharge tube containing gas at low pressure was used.
• Two metal plates acted as the cathode (-) and the anode (+).
• A high voltage was applied across the electrodes.
• Rays produced inside the tube travelled from the cathode towards the anode.

Observations

• Rays originated from the cathode.
• They travelled in straight lines.
• The rays produced a glowing spot on the screen placed inside the tube.
• They were deflected by electric and magnetic fields.
• The behaviour of the rays was the same irrespective of the gas present in the tube.

Conclusion

Thomson concluded that these rays consist of tiny negatively charged particles present in all atoms. These particles were named electrons. 

Properties of Electrons

• Negative Charge – Every electron carries one unit negative charge.
• Very Small Mass – The mass of an electron is much smaller than that of a proton or neutron.
• Present in Every Atom – Electrons are common to the atoms of all elements.
• Move Around the Nucleus – Electrons occupy regions around the nucleus called shells or energy levels.

Thomson's Atomic Model

After discovering the electron, J. J. Thomson proposed the first model describing the internal structure of the atom.

Main Features of Thomson's Model

• The atom is a uniform sphere of positive charge.
• Negatively charged electrons are embedded throughout the positively charged sphere.
• The total positive charge equals the total negative charge, making the atom electrically neutral.
• Electrons are distributed throughout the atom like raisins embedded in a pudding.

Plum Pudding Model

Thomson's model is popularly known as the Plum Pudding Model because it compares the atom to a pudding in which negatively charged electrons are embedded in a positively charged material.

Merits of Thomson's Model

• It was the first model to include subatomic particles inside the atom.
• It explained why atoms are electrically neutral.
• It established that atoms are divisible and contain smaller particles.

Limitations of Thomson's Model

• It could not explain the presence of a dense central nucleus.
• It failed to describe the actual arrangement of electrons.
• It could not explain the results of Rutherford's gold foil experiment.
• It did not explain the stability of atoms.

Importance of the Discovery of Electron

• It proved that atoms contain smaller particles.
• It marked the beginning of modern atomic structure.
• It led to the development of improved atomic models.
• It helped scientists understand electricity and chemical bonding.

ATP Education Concept Builder

The discovery of the electron changed the idea that atoms were indivisible. Thomson's model was the first attempt to explain the internal structure of the atom. Although it was later replaced by better models, it played an important role in the development of atomic theory.

ATP Education Exam Booster

• Remember the cathode ray experiment and its observations.
• Learn the properties of electrons.
• Understand the main features of Thomson's Plum Pudding Model.
• Remember the merits and limitations of Thomson's model.
• CBSE frequently asks competency-based questions on how the discovery of the electron changed Dalton's atomic theory.

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