Lesson 1 of 12 · 9 min

Discovery of sub-atomic particles

NCERT §2.1

The yellow street lamp outside your window is a glass tube of gas with a high voltage across it. Over a century ago, tubes almost like it showed that atoms have parts.

The story this chapter follows: Diwali lamps and a hydrogen tube

On Diwali night a sodium street lamp glows yellow at 589 nm while rockets burst red, green and yellow overhead. The next morning, in the school lab, a hydrogen discharge tube glows pink and a hand spectroscope splits it into four sharp lines. The lamp's numbers (5.09 × 10¹⁴ Hz, 2.11 eV per photon, 203 kJ per mole) and hydrogen's red line at 656 nm run through the chapter, from the first discharge tube to the last electron in an orbital box.

The lesson in notes

In short

Discharge through gases at very low pressure and very high voltage produces cathode rays, a stream that travels from the cathode to the anode; it is made visible where it strikes a coating such as zinc sulphide.

Cathode rays go straight in the absence of fields and bend in electric or magnetic fields as negative particles would, so they are negatively charged particles, the electrons.

The properties of cathode rays do not change with the metal of the electrodes or the gas in the tube, so electrons are a common part of every atom.

J.J. Thomson (1897) balanced perpendicular electric and magnetic fields to find e/mₑ = 1.758820 × 10¹¹ C kg⁻¹. Deflection grows with charge and field strength and falls as mass rises.

Millikan's oil drop experiment (1906-14) showed every droplet charge is a whole-number multiple of e (q = ne). The accepted value is e = 1.602176 × 10⁻¹⁹ C; combined with e/mₑ it gives mₑ = 9.1094 × 10⁻³¹ kg.

Canal rays are positive gaseous ions: unlike cathode rays, their mass and e/m depend on the gas, some carry more than one unit of charge, and they deflect the opposite way to electrons.

The lightest positive ion, from hydrogen, is the proton (characterised in 1919). Chadwick (1932) found the neutral neutron, slightly heavier than the proton, by bombarding a thin beryllium sheet with α-particles.

Masses in u: electron 0.00054, proton 1.00727, neutron 1.00867. Relative charges are −1, +1 and 0.