Discovery of Electron, Proton and Neutron — Revision Notes

⚡ 30-Second Revision

Electron: — Discovered by J.J. Thomson (1897) via Cathode Ray Exp. Charge $e = -1.602 \times 10^{-19},\text{C}$ (relative -1). Mass $m_e = 9.109 \times 10^{-31},\text{kg}$ (relative $1/1837$ of H atom). Universal constituent.
Proton: — Discovered by Goldstein (canal rays, 1886) & Rutherford (1919). Charge $e = +1.602 \times 10^{-19},\text{C}$ (relative +1). Mass $m_p = 1.672 \times 10^{-27},\text{kg}$ (relative 1 amu). In nucleus.
Neutron: — Discovered by James Chadwick (1932) via Beryllium bombardment. Charge $0$. Mass $m_n = 1.674 \times 10^{-27},\text{kg}$ (relative 1 amu). In nucleus.

2-Minute Revision

The atom, once thought indivisible, was found to comprise subatomic particles. The electron was discovered by J.J. Thomson in 1897 using cathode ray tubes. He observed negatively charged rays, whose properties were independent of the gas or electrode, proving electrons are universal constituents of matter.

He determined their charge-to-mass ($e/m$) ratio. Robert Millikan later measured the electron's charge, allowing its mass calculation. The proton was first observed as 'canal rays' (positive ions) by Eugen Goldstein in 1886.

Ernest Rutherford, through his gold foil experiment and subsequent work, identified the fundamental positive particle in the nucleus as the proton in 1919. Protons carry a positive charge equal in magnitude to the electron's negative charge but are much heavier.

Finally, the neutron was discovered by James Chadwick in 1932. He bombarded beryllium with alpha particles, observing highly penetrating, neutral radiation that ejected protons from paraffin. This led him to conclude the existence of a neutral particle with a mass similar to a proton, completing the basic atomic model.

5-Minute Revision

The journey to understanding the atom's structure began with the discovery of its fundamental constituents: electrons, protons, and neutrons. Electrons were discovered by J.J. Thomson in 1897 through his cathode ray experiments.

In a discharge tube, he observed rays emanating from the cathode that were deflected by electric and magnetic fields towards the positive plate, indicating a negative charge. Crucially, the properties of these cathode rays (their $e/m$ ratio) were independent of the electrode material or the gas in the tube, establishing electrons as universal, fundamental particles.

Their charge is $-1.602 \times 10^{-19},\text{C}$ and mass is $9.109 \times 10^{-31},\text{kg}$.

The proton's discovery followed. Eugen Goldstein, in 1886, observed 'canal rays' (positive ions) moving in the opposite direction to cathode rays in a discharge tube with a perforated cathode. Unlike cathode rays, the properties of canal rays depended on the gas used.

Ernest Rutherford later identified the fundamental positive particle, the proton, in 1919, primarily through his work on the atomic nucleus. Protons carry a charge of $+1.602 \times 10^{-19},\text{C}$ and have a mass of $1.

672 imes 10^{-27}, ext{kg}$, approximately 1836 times that of an electron.

The neutron was the last to be discovered, by James Chadwick in 1932. Scientists had hypothesized a neutral particle in the nucleus to account for atomic mass discrepancies. Chadwick bombarded beryllium with alpha particles, observing a highly penetrating, uncharged radiation.

When this radiation struck paraffin wax, it ejected protons. By applying conservation laws, Chadwick deduced that this radiation consisted of neutral particles with a mass almost identical to a proton ($1.

674 imes 10^{-27}, ext{kg}$). Neutrons are electrically neutral and reside in the nucleus with protons. These discoveries collectively formed the basis of the modern atomic model.

Prelims Revision Notes

Discovery of Electron

Discoverer: — J.J. Thomson (1897)
Experiment: — Cathode Ray Experiment (Discharge tube)
Observations:

* Rays from cathode to anode. * Travel in straight lines. * Produce fluorescence. * Deflected by electric (towards positive plate) and magnetic fields. * Rotate paddle wheel (possess mass & kinetic energy). * Crucial: Properties (e.g., $e/m$ ratio) independent of cathode material or gas in tube.

Conclusion: — Cathode rays are streams of negatively charged particles (electrons), universal constituents of all atoms.
Properties:

* Charge ($e$): $-1.602 \times 10^{-19},\text{C}$ (relative $-1$) * Mass ($m_e$): $9.109 \times 10^{-31},\text{kg}$ (relative $approx 1/1837$ of H atom) * $e/m$ ratio: $1.7588 \times 10^{11},\text{C/kg}$

Discovery of Proton

Discoverer: — Eugen Goldstein (1886) observed canal rays; Ernest Rutherford (1919) identified the proton.
Experiment: — Anode Ray Experiment (Discharge tube with perforated cathode).
Observations:

* Rays from anode to cathode, passing through perforations. * Travel in straight lines, opposite to cathode rays. * Deflected by electric and magnetic fields (opposite to cathode rays, indicating positive charge). * Crucial: Properties (e.g., $e/m$ ratio) depend on the nature of the gas in the tube.

Conclusion: — Canal rays are streams of positive ions. The lightest positive ion ($H^+$ from hydrogen gas) was identified as the proton.
Properties:

* Charge ($e$): $+1.602 \times 10^{-19},\text{C}$ (relative $+1$) * Mass ($m_p$): $1.672 \times 10^{-27},\text{kg}$ (relative $approx 1$ amu)

Discovery of Neutron

Discoverer: — James Chadwick (1932)
Experiment: — Bombardment of Beryllium ($^9_4\text{Be}$) with alpha particles ($^4_2\text{He}$). Reaction: $^9_4\text{Be} + ^4_2\text{He} \rightarrow ^{12}_6\text{C} + ^1_0\text{n}$
Observations:

* Highly penetrating radiation produced. * Not deflected by electric or magnetic fields (electrically neutral). * Ejected high-energy protons from paraffin wax.

Conclusion: — The radiation consisted of neutral particles with mass similar to a proton, named neutrons.
Properties:

* Charge: $0$ * Mass ($m_n$): $1.674 \times 10^{-27},\text{kg}$ (relative $approx 1$ amu, slightly heavier than proton) * Location: Inside the nucleus along with protons.

Vyyuha Quick Recall

To remember the order and discoverers: Every Teacher Praises Good Results, Never Cheat.

Electron - Thomson
Proton - Goldstein (observed canal rays), Rutherford (identified proton)
Neutron - Chadwick