Sub-atomic Particles
Learning Objective: Understand how the electron, proton, and neutron were discovered, and how their properties (charge, mass) were measured.
Quick Notes:
- Atoms are not indivisible, they contain sub-atomic particles: electrons, protons, and neutrons.
- Electron was discovered by J.J. Thomson via cathode ray experiments.
- Charge-to-mass ratio (e/m) for electrons was measured by Thomson: e/m = 1.758820 × 1011 C kg−1
- Charge of the electron measured by Millikan’s oil drop experiment: e = −1.6022 × 10−19 C
- Proton was discovered by Goldstein using canal rays.
- Neutron was discovered by James Chadwick in 1932 – neutral charge and mass ≈ proton.
Full Notes:
2.1.1 Discovery of Electron
Background:
Before the electron’s discovery, atoms were thought to be indivisible. However, experiments with electric discharge tubes revealed the presence of particles smaller than atoms.
Cathode Ray Experiment (J.J. Thomson, 1897):
Thomson passed high voltage electricity through gases at low pressure (~10−4 atm) in a discharge tube with metal electrodes.
Observations:
- Rays originated from the cathode (negative electrode) and travelled to the anode.
- These rays:
- Were negatively charged (deflected by electric/magnetic fields).
- Caused a fluorescent glow on hitting the glass.
- Moved a paddle wheel, showing they had mass.
- Were produced regardless of the gas or cathode material.
Conclusion:
The rays consisted of negatively charged particles, later named electrons.
2.1.2 Charge to Mass Ratio of Electron
Thomson calculated the specific charge (e/m) of the electron by applying electric and magnetic fields to balance the deflection of cathode rays.
(Equation 2.1)
This value was consistent for all elements, proving that electrons are universal constituents of matter.
2.1.3 Charge on the Electron
While Thomson gave e/m, the exact charge was later measured by Robert Millikan in his oil drop experiment (1909).
Millikan’s Oil Drop Experiment:
Millikan sprayed fine oil droplets into a chamber between two charged plates. By adjusting the electric field, he suspended a charged droplet in equilibrium against gravity. By measuring:
- The electric field needed to suspend the droplet
- The droplet’s mass and fall rate (determined by its size and terminal velocity)
He calculated the charge on a single electron:
e = −1.6022 × 10−19 coulombs
Using Thomson’s e/m and Millikan’s e:
Mass of electron (m) = e / (e/m)
= (1.6022 × 10−19) / (1.758820 × 1011)
= 9.1094 × 10−31 kg
me = e / (e / me)
= (1.602176 × 10−19 C) / (1.758820 × 1011 C kg−1)
= 9.1094 × 10−31 kg (Equation 2.2)
2.1.4 Discovery of Protons and Neutrons
Discovery of Protons (Goldstein, ~1886)
Goldstein used a modified discharge tube with a perforated cathode and observed rays traveling in the opposite direction to cathode rays.
These rays:
- Were called canal rays (or anode rays)
- Consisted of positive particles
- Had properties dependent on the gas used, unlike cathode rays
In hydrogen gas, the positively charged particle observed had:
Charge = +1.6022 × 10−19 C
Mass = 1.6726 × 10−27 kg
This particle was named the proton, and it is about 1836 times heavier than the electron.
Discovery of Neutrons (James Chadwick, 1932)
The mass of hydrogen (~1 amu) did not explain the greater masses of other atoms (e.g. helium ≈ 4 amu, not 2). There had to be another neutral particle.
James Chadwick bombarded beryllium with α-particles, which emitted uncharged radiation capable of knocking protons out of paraffin wax.
These particles:
- Had no charge
- Had mass similar to protons
- Were named neutrons
Neutron mass = 1.6750 × 10−27 kg ≈ 1 amu
Neutrons reside in the nucleus and contribute to atomic mass, but not to charge.
Particle Summary
| Particle | Symbol | Charge (C) | Mass (kg) |
|---|---|---|---|
| Electron | e− | −1.6022 × 10−19 | 9.1094 × 10−31 |
| Proton | p+ | +1.6022 × 10−19 | 1.6726 × 10−27 |
| Neutron | n0 | 0 | 1.6750 × 10−27 |
Millikan discovered charge of electron and Thomson measured e/m. Protons are gas-dependent in canal rays whereas electrons are universal. Neutron discovery is tied to nuclear reactions, not electric or magnetic deflections.
Summary
- Electrons were discovered via cathode rays and have measurable e/m and charge.
- Millikan’s oil drop provided the elementary charge enabling electron mass calculation.
- Protons were identified in canal rays and carry positive charge with higher mass.
- Neutrons were discovered by Chadwick as neutral nuclear particles with mass ≈ 1 amu.
- Sub-atomic particles explain atomic structure and account for mass and charge of atoms.