Development of the model of the atom
The atomic model has changed dramatically as scientists collected new evidence. Each model was the best fit at the time but was overturned by new data — a classic example of how science progresses.
1. Dalton's solid sphere (~1803)
John Dalton proposed atoms as tiny, indivisible spheres. Different elements had different atoms. This explained why chemicals combined in fixed ratios.
2. JJ Thomson's plum pudding (1897)
Thomson discovered the electron in cathode rays. He proposed the plum pudding model: a "pudding" of positive charge, with negative electrons embedded like raisins. The atom is neutral overall.
3. Rutherford's nuclear model (1909, the alpha-scattering experiment)
Hans Geiger and Ernest Marsden, under Rutherford, fired alpha particles (positive helium nuclei) at thin gold foil. They expected slight deflections (consistent with plum pudding). They observed:
- Most alphas passed through almost undeflected → atoms are mostly empty space.
- A few were deflected at large angles → there's a small positive region.
- A very few bounced almost straight back → that region is dense, massive and positive.
Conclusion: the atom has a tiny, positively charged nucleus at its centre. Electrons orbit around it.
4. Bohr's electron shells (1913)
Niels Bohr added that electrons are in fixed energy levels (shells), not just any orbit. Light absorbed or emitted by atoms comes from electrons jumping between these discrete levels — explaining the line spectra of elements.
5. Discovery of the neutron (1932)
James Chadwick discovered the neutron. Atomic masses now made sense (A − Z particles in the nucleus that aren't protons).
6. The modern picture
Today's quantum model treats electrons as standing waves in orbitals rather than tidy circular orbits. For GCSE, the Bohr-with-neutrons picture is sufficient.
Why models change
This story is the centrepiece of "How Science Works" at GCSE: when evidence contradicts a model, the scientific community revises or replaces it. Models are tools for explanation and prediction — not fixed truths.
⚠Common mistakes
- Confusing Thomson's discovery (the electron) with Rutherford's (the nucleus).
- Saying the alpha-scattering experiment "proved" the nucleus exists — it gave evidence; nothing in science is finally proved.
- Forgetting to mention Bohr's contribution of electron shells.
- Crediting Rutherford with the neutron — it was Chadwick.
AI-generated · claude-opus-4-7 · v3-deep-physics