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).

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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.

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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: 1.675×10⁻²⁷kg (slightly more than the mass of a proton).

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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).

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Summary of Subatomic Particles:

Particle	Symbol	Charge	Mass	Discovered By	Year
Electron	e−	Negative (−1)	9.1×10−31 kg	J.J. Thomson	1897
Proton	p+	Positive (+1)	1.67×10−27 kg	Ernest Rutherford	1919
Neutron	n0	Neutral (0)	1.675×10−27 kg	James Chadwick	1932