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Q.

Explain the discovery of electron by Thomson?

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Detailed Solution

J.J. Thomson discovered the electron in 1897 through his experiments with cathode rays. J.J. Thomson, a British physicist, discovered the electron while experimenting with cathode ray tubes. In these tubes, when a high voltage was applied, a beam of particles was emitted from the cathode. These particles traveled in straight lines and could cast shadows, showing that they were real particles.

Thomson observed that the cathode rays were attracted to a positively charged plate and repelled by a negatively charged one. This behavior indicated that the rays were made of negatively charged particles. He named these particles “electrons.”

Details of the Experiment

  • Thomson used a glass tube with electrodes at both ends.
  • He applied a high voltage between the electrodes in a vacuum.
  • A greenish glow appeared on the opposite end of the cathode, indicating the presence of cathode rays.
  • When an electric or magnetic field was applied, the rays bent toward the positive plate, proving the particles were negatively charged.

Significance of Thomson’s Discovery

  • Proved that atoms are not indivisible as previously thought.
  • Introduced the concept of subatomic particles.
  • Led to the development of the "plum pudding model" of the atom, where electrons were embedded in a positively charged sphere.

Thomson also calculated the charge-to-mass ratio (e/m) of the electron using deflection techniques. Although he could not measure the mass or charge separately, this work laid the foundation for future discoveries in atomic structure.

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