Discovery of Electron, Proton, and Neutron
The journey from Dalton's indivisible atom to the modern understanding of subatomic particles.
The discovery of fundamental subatomic particles—electrons, protons, and neutrons—revolutionized our understanding of matter. This article details the key experiments performed by J.J. Thomson, Rutherford, and James Chadwick that peeled back the layers of the atom.
1. Discovery of Electron ($e^-$)
Discoverer: J.J. Thomson (1897)
Experiment: Cathode Ray Tube (CRT) Experiment
J.J. Thomson studied the conduction of electricity through gases at very low pressure. He observed that when high voltage (~10,000 V) was applied across electrodes in a discharge tube, a stream of particles moved from the cathode (negative) to the anode (positive). These were named Cathode Rays.
Key Properties of Cathode Rays:
- They travel in straight lines.
- They consist of negatively charged material particles (deflected towards positive plate in electric fields).
- Crucial Finding: The charge-to-mass ratio ($e/m$) of cathode ray particles is independent of the nature of the gas or the electrode material. This proved that electrons are universal constituents of all matter.
Charge to Mass Ratio:
Later, R.A. Millikan determined the exact charge of an electron via the Oil Drop Experiment ($ -1.602 \times 10^{-19} C $).
2. Discovery of Proton ($p^+$)
Discoverer: Eugen Goldstein (observed rays), Ernest Rutherford (characterized/named)
Experiment: Modified Discharge Tube (Canal Rays)
In 1886, Goldstein used a perforated cathode in a discharge tube and observed a new type of ray originating from the anode and passing through the holes (canals) of the cathode. These were called Anode Rays or Canal Rays.
Key Properties of Anode Rays:
- They are positively charged.
- Unlike electrons, the nature (charge and mass) of these particles depends on the gas present in the tube.
- The lightest positive ion was obtained from Hydrogen gas and was named the Proton ($H^+$) by Rutherford in 1919.
3. Discovery of Neutron ($n^0$)
Discoverer: James Chadwick (1932)
Experiment: Bombardment of Beryllium with Alpha Particles
Even after the discovery of electrons and protons, the atomic mass could not be fully explained (it was higher than the mass of protons alone). Rutherford predicted the existence of a neutral particle.
In 1932, James Chadwick bombarded a thin sheet of Beryllium with $\alpha$-particles. He observed the emission of highly penetrating neutral radiation that was not deflected by electric or magnetic fields.
Nuclear Reaction:
Comparison of Particles
| Particle | Discoverer | Charge (C) | Relative Mass |
|---|---|---|---|
| Electron | J.J. Thomson | -1.602 × 10-19 | 1/1837 u |
| Proton | Goldstein/Rutherford | +1.602 × 10-19 | 1 u |
| Neutron | Chadwick | 0 | 1 u (heavier than p+) |
Knowledge Check
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