Discovery of Subatomic Particles: Class 11 Chemistry Notes with Features

Subatomic particles are atomic components that make up an atom. These constituents are present in all ordinary matter. The discovery of subatomic particles led scientists to classify these building blocks based on their electric charges and locations. Protons, which contain a positive charge, and neutrons with no charge reside within the nucleus of the atom, while the negatively charged electrons orbit around the nucleus. 
The next sections below talk about how scientists discovered these subatomic particles, and the experiments they used that revolutionised the way we view the world at both macroscopic and microscopic levels. 

The electron is a negatively charged subatomic particle with a small mass. This was identified through experiments that involved electric and magnetic fields acting on charged particles in gases.

Cathode Ray Experiment

It was J.J. Thomson (1897) who discovered electrons by studying cathode rays in a discharge tube.

These rays were produced at low pressure under high voltage.

Electric and magnetic fields deflected them. That indicated there are negatively charged particles.

Thomson measured the charge-to-mass ratio of electrons. That led him to establish them as universal constituents of matter.

Key Features of Electrons Based on the Cathode Ray Experiment

• Charge of an electron: $-1.602\times {10}^{-19}\mathrm{C}$ .
• Mass of an electron: $9.109\times {10}^{-31}\mathrm{k}\mathrm{g}$ (approximately $1/1836$ that of a proton).
• Charge-to-mass ratio of the electron ( $e/m$ ): $\frac{e}{m}=1.7588\times {10}^{11}\mathrm{C}/\mathrm{k}\mathrm{g}$

This is calculated using: $\frac{e}{m}=\frac{E}{B^{2}r}$ where $E$ is the electric field strength, $B$ is the magnetic field, and $r$ is the radius of the particle's path.

• Millikan's oil drop experiment (1909) confirmed the charge of the electron.

Experimental Insight of J.J. Thomson's Cathode Ray Experiment

• Cathode rays moved toward the anode, glowed on striking a fluorescent screen, and cast shadows, proving their particle nature.
• Deflection in fields confirmed the presence of a negative charge.

JEE questions may focus on calculating $e/m$ or interpreting cathode ray behaviour. Have a look at the JEE Main paper analysis for this. 

The proton is a positively charged subatomic particle residing in the nucleus. This subatomic particle was discovered through experiments that analysed positively charged ions in gas discharge tubes.

Goldstein's Experiment

We owe this discovery of subatomic particles to Eugen Goldstein.

In 1886, he observed canal rays (positive rays) in a perforated cathode discharge tube.

These rays moved in the opposite direction to cathode rays.

Wilhelm Wien and Thomson later analysed these rays. They could identify the proton as a fundamental particle with a positive charge equal in magnitude to that of the electron.

Key Features of Protons based on the Goldstein Experiment

• Charge of the proton: $+1.602\times {10}^{-19}\mathrm{C}$ .
• Mass of the proton: $1.6726\times {10}^{-27}\mathrm{k}\mathrm{g}$ (1836 times heavier than an electron).
• Charge-to-mass ratio of the proton: $\frac{e}{m}=9.5788\times {10}^7\mathrm{C}/\mathrm{k}\mathrm{g}$
• Found in all atomic nuclei. This determines the atomic number.

Goldstein's Experimental Insight for Protons

• Canal rays varied with the type of gas, indicating a dependence on atomic mass.
• Deflection in magnetic fields confirmed the presence of a positive charge.

JEE problems may involve proton properties or canal ray experiments.

The neutron is a neutral subatomic particle in the nucleus. This subatomic particle was identified through experiments of nuclear reactions and particle bombardment.

James Chadwick’s Neutron Discovery Experiment

In 1932, James Chadwick discovered neutrons by bombarding beryllium with alpha particles.

That produced uncharged particles with a mass similar to protons.

These particles were unaffected by electric or magnetic fields. And, that could explain nuclear mass discrepancies.

Key Features of Neutrons

• Charge of neutron: 0.
• Mass of a neutron: $1.6749\times {10}^{-27}\mathrm{k}\mathrm{g}$ (slightly heavier than a proton).
• Reaction example of a neutron: ${}_4^9\mathrm{B}\mathrm{e}+{}_2^4\alpha \to {}_6^{12}\mathrm{C}+{}_0^1\mathrm{n}$
• Contributes to nuclear stability and atomic mass.

Experimental Insight for Neutron's Discovery

• Neutrons penetrated paraffin, ejecting protons, which were detected by ionisation chambers.
• Neutrality is confirmed by the lack of deflection in fields.

Exam questions may test properties of neutrons or the balancing of nuclear reactions.

Have a look at this table of comparison for protons, electron, and neutron. 

You can expect questions of this type in competitive tests covering the Structure of Atoms chapter. You can see that after you are clear with the experiments and calculations that are used for the discovery of subatomic particles, you will be tested on application-based questions. They are not just based on JEE Mains, but also for NEET exams. 

Example 1: Calculate the charge-to-mass ratio of an electron in a cathode ray tube with an electric field $E=2\times {10}^4\mathrm{N}/\mathrm{C}$ , magnetic field $B=0.02\mathrm{T}$ , and path radius $r=0.03\mathrm{m}$ .

Solution:

$\frac{e}{m}=\frac{E}{B^{2}r}=\frac{2\times {10}^4}{(0.02{)}^2\times 0.03}=\frac{2\times {10}^4}{1.2\times {10}^{-5}}\approx 1.67\times {10}^{11}\mathrm{C}/\mathrm{k}\mathrm{g}$

Example 2: In Chadwick's experiment, an alpha particle $\left({}_2^4\mathrm{H}\mathrm{e}\right)$ bombards beryllium $\left({}_4^9\mathrm{B}\mathrm{e}\right)$ to produce a neutron and another nucleus. Identify the nucleus. 

Solution:

Reaction: ${}_4^9\mathrm{B}\mathrm{e}+{}_2^4\mathrm{H}\mathrm{e}\to {}_0^1\mathrm{n}+X$

Balancing atomic and mass numbers: $9+4=1+A,\mathrm{}A=12;\mathrm{}4+2=0+Z,\mathrm{}Z=6$

Nucleus: ${}_6^{12}\mathrm{C}$ .

You can quickly go through these facts. 

1. Electron: Negatively charged, discovered via cathode rays (Thomson, 1897).
2. Proton: Positively charged, identified through canal rays (Goldstein, 1886).
3. Neutron: Neutral, found via nuclear reactions (Chadwick, 1932).
4. Experiments: Cathode rays, canal rays, and alpha bombardment revealed the properties of particles.