DISCOVERY OF SUB-ATOMIC PARTICLES i.e. ELECTRON, PROTON AND NEUTRON

The discovery of subatomic particles—electron, proton, and neutron

1. Discovery of the Electron

  • Discovered By: J.J. Thomson in 1897
  • Experiment: Cathode Ray Tube Experiment
    • Thomson passed electricity through gases at low pressure in a sealed glass tube (cathode ray tube).
    • He observed a beam of particles (cathode rays) traveling from the negatively charged electrode (cathode) to the positively charged electrode (anode).
    • These rays were deflected by electric and magnetic fields, indicating they were negatively charged.
  • Conclusion:
    • Cathode rays were made up of negatively charged particles, later named electrons.
    • He proposed the “plum pudding model” of the atom, where electrons were embedded in a positively charged “pudding-like” sphere.
  • Properties of Electrons:
    • Charge: Negative (−1.6×10−19 C).
    • Mass: 9.1×10−31 kg (about 1/1836 the mass of a proton).

2. Discovery of the Proton

  • Discovered By: Eugen Goldstein (1886) and later clarified by Ernest Rutherford in 1919.
  • Experiment:
    • Goldstein’s Canal Ray Experiment:
      • Using a modified cathode ray tube, Goldstein observed rays traveling in the opposite direction of cathode rays, which he called canal rays or positive rays.
      • These rays were found to be positively charged.
    • Rutherford’s Contribution:
      • In 1919, Rutherford proved that these positive particles (protons) were part of the atom.
      • He observed that when alpha particles were fired at nitrogen gas, hydrogen nuclei (protons) were ejected, showing that hydrogen atoms (protons) were a fundamental part of other atoms.
  • Conclusion:
    • Protons are positively charged subatomic particles.
    • Charge: Positive (+1.6×10−19 C).
    • Mass: 1.67×10−27 kg.

3. Discovery of the Neutron

  • Discovered By: James Chadwick in 1932
  • Experiment:
    • Chadwick bombarded beryllium with alpha particles and observed that a highly penetrating radiation was emitted.
    • This radiation was not deflected by electric or magnetic fields, indicating it was neutral (no charge).
    • Further experiments revealed that these neutral particles had a mass almost equal to that of a proton.
  • Conclusion:
    • Neutrons are neutral subatomic particles found in the nucleus of an atom.
    • Charge: Zero (neutral).
    • Mass: 1.675×10−27 kg (slightly more than the mass of a proton).

Milestones in Atomic Theory:

  1. Dalton’s Atomic Theory (1803):
    • Atoms are indivisible particles.
    • This idea was later revised after the discovery of subatomic particles.
  2. J.J. Thomson’s Model (1897):
    • “Plum pudding model”: Electrons embedded in a sphere of positive charge.
  3. Rutherford’s Nuclear Model (1911):
    • Discovered the nucleus by firing alpha particles at gold foil.
    • Concluded that atoms have a dense, positively charged nucleus surrounded by electrons.
  4. Bohr’s Model (1913):
    • Suggested that electrons orbit the nucleus in fixed energy levels.
  5. Chadwick’s Neutron Discovery (1932):
    • Explained the existence of isotopes (atoms with the same number of protons but different numbers of neutrons).

Summary of Subatomic Particles:

ParticleSymbolChargeMassDiscovered ByYear
Electrone−Negative (−1)9.1×10−31 kgJ.J. Thomson1897
Protonp+Positive (+1)1.67×10−27 kgErnest Rutherford1919
Neutronn0Neutral (0)1.675×10−27 kgJames Chadwick1932

Similar Posts

Leave a Reply

Your email address will not be published. Required fields are marked *