CHEMISTRY · Stage 8 · Chapter 7: Inside Atoms

Chemistry 7.2 Discovering electrons — J.J. Thomson and the Plum Pudding Model

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Lesson summary

7.2 Discovering electrons — J.J.

Part 1 of 8Early ideas of the atom

Dalton's model — in 1803 John Dalton suggested that all matter is made of tiny, solid, indivisible spheres called atoms, with a different kind for each element.
- For nearly a century the atom was thought to be the smallest particle, like a tiny billiard ball.

Gas tubes — in the 1800s scientists applied huge voltages across sealed tubes containing tiny amounts of gas.
- The gases conducted electricity.
Hittorf's observation — Johann Hittorf noticed a green glow on the screen, caused by rays from the negative electrode.
Cathode rays = rays from the negative electrode that travel through the gas when the voltage is high and make the screen fluoresce.

Scientific question — Thomson asked what cathode rays are.
Hypothesis — cathode rays move towards a positive electrode, so they might have a negative charge.
Prediction — if rays pass between electrically charged metal plates, they will bend towards the positively charged metal.
Observation — the rays bent towards the positively charged metal, so the prediction was correct.
Conclusion — cathode rays are charged, and the charge is negative.
Further work — more investigations showed the rays are made of particles with the same tiny mass and same charge.

Further evidence — the rays were deflected by electric and magnetic fields.
- The particles had a mass about 1/1800 of the lightest atom, hydrogen.
- Their properties did not depend on the metal of the electrode, showing they are part of all atoms.

Electron — by 1897 Thomson had discovered the first sub-atomic particle, later named the electron.
Atoms are divisible — atoms were therefore not indivisible.

Evidence used — electrons are negatively charged but atoms have no overall charge, so there must be positive charge to cancel them.
Model — an atom is a positively charged sphere with negative electrons embedded throughout it.
- The positive charge is spread out evenly so the whole atom is neutral.
Explains — it explains the cathode ray observations, why atoms are neutral and why electrons can be removed, as in static electricity.

Analogythe atom is compared to a plum pudding, with electrons as the plums.
Strengthit shows small electrons spread through a larger body, and the idea of embedded parts.
Weaknessplums have no charge, but electrons are negatively charged.
Weaknessthe pudding has no charge, but Thomson's sphere is positively charged.

Limits of the modelit could not explain the arrangement of positive charge or the results of later experiments.
Gold foilin 1909 Geiger and Marsden fired alpha particles at gold foil and some bounced back, which did not match the plum pudding model.
ReplacementRutherford used this evidence to replace it with the nuclear model, showing that models change when new evidence appears.

Model	Scientist	Idea
Solid sphere	Dalton	Atoms are indivisible balls
Plum pudding	Thomson	Electrons embedded in a positive sphere

Cathode ray tube — a beam of electrons in a vacuum tube is steered by electric or magnetic fields onto a fluorescent screen that glows where the beam hits.
- Old televisions and oscilloscopes used this idea.
Electronics — the discovery of the electron began the field of electronics.
Nobel Prize — Thomson received a Nobel Prize in 1906.

Last updated: 13 October 2026