Chapter 2 / 2.1 Discovery Of Sub-Atomic Particles

🔍 Discovery of Sub-Atomic Particles

Scientists learned about atoms by studying electricity in gases! Remember this rule: Like charges repel, unlike charges attract ⚡➡️⚡

Michael Faraday (1830) found electricity causes chemical reactions in solutions, hinting electricity is made of particles.
Later, scientists used cathode ray tubes (glass tubes with 2 metal electrodes) at low pressure + high voltage.
Cathode rays flowed from negative (cathode) to positive (anode). Key properties:
- Invisible but make fluorescent materials glow ✨ (like old TV screens!)
- Travel in straight lines without fields 📏
- Deflected by electric/magnetic fields like negative particles ⬅️
- Same behavior for any electrode material or gas → electrons are in all atoms!

Used electric + magnetic fields at right angles to each other and electron path.
Found deflection depends on:
- Particle’s charge (higher charge = more deflection)
- Particle’s mass (lighter = more deflection)
- Field strength (stronger field = more deflection)
Calculated the ratio: \[\frac{e}{m_e} = 1.758820 \times 10^{11} \text{C kg}^{-1}\] where \(e\) = charge, \(m_e\) = mass of electron.

Oil droplets became charged by colliding with ions in air.
By balancing gravitational, electric, and drag forces:
- Charge on droplets was always a multiple of a fundamental charge: \(q = n e\)
Found electron charge: \(e = -1.602176 \times 10^{-19} \text{C}\)
Used Thomson’s \(\frac{e}{m_e}\) to find electron mass: \[m_e = \frac{e}{e/m_e} = 9.1094 \times 10^{-31} \text{kg}\]

Canal rays (positively charged particles) were found in modified cathode tubes:
- Mass depends on gas used → positive ions of the gas
- Deflect opposite to electrons in fields ⮕
- Lightest particle (from hydrogen) = proton (1919)
Neutrons (neutral particles) discovered by Chadwick (1932):
- Shot α-particles at beryllium → emitted particles slightly heavier than protons
- No charge → named neutrons!

🚀 Cathode ray properties: From cathode to anode, deflect as negative particles, independent of gas/electrode.
🧲 Thomson’s \(e/m_e\) experiment: How fields deflect electrons + value of \(\frac{e}{m_e}\).
💧 Millikan’s oil drop: Charge quantization (\(q = ne\)) + electron charge (\(e\)) calculation.
⚛️ Protons vs. canal rays: Protons are smallest positive ions (from H⁺); canal rays are gas-dependent positive ions.
⏺️ Neutron discovery: Chadwick’s α-particle + beryllium experiment → neutral particles heavier than protons.