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.
- Goldstein’s Canal Ray Experiment:
- 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:
- Dalton’s Atomic Theory (1803):
- Atoms are indivisible particles.
- This idea was later revised after the discovery of subatomic particles.
- J.J. Thomson’s Model (1897):
- “Plum pudding model”: Electrons embedded in a sphere of positive charge.
- 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.
- Bohr’s Model (1913):
- Suggested that electrons orbit the nucleus in fixed energy levels.
- 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:
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 |