Scientific models are the best current explanation of how something works — but they must change when new experimental evidence cannot be explained by the existing model.
The history of atomic models is a perfect example of how science works:
A model is proposed → experiments are done → if results conflict with the model, the model is revised.
The key models in chronological order:
1. Dalton's solid sphere (1803)
2. Thomson's plum pudding model (1897)
3. Rutherford's nuclear model (1911)
4. Bohr's shell model (1913)
From Solid Sphere to Nuclear Model
DALTON'S MODEL (1803):
Atoms are solid, indivisible spheres.
Different elements have differently sized spheres.
Atoms cannot be split, created or destroyed.
Revised when electrons were discovered.
THOMSON'S PLUM PUDDING MODEL (1897):
Discovery of the ELECTRON — a tiny negatively charged particle inside atoms — proved atoms were not solid or indivisible.
Thomson proposed: an atom is a ball of POSITIVE CHARGE with ELECTRONS dotted throughout — like plums in a pudding (or currants in a bun).
Revised after Rutherford's gold foil experiment.
RUTHERFORD'S NUCLEAR MODEL (1911):
Rutherford fired positively charged ALPHA PARTICLES at a very thin sheet of gold foil.
Expected (plum pudding): all particles would pass through with small deflections.
Observed:
MOST particles passed STRAIGHT THROUGH — atoms are mostly empty space.
SOME were deflected at large angles — something positive inside repelled them.
A FEW bounced STRAIGHT BACK — hit something very dense and concentrated.
Conclusion: atoms have a tiny, dense, positively charged NUCLEUS at the centre, surrounded by mostly empty space with electrons.
BOHR'S MODEL (1913):
Bohr proposed electrons orbit in fixed SHELLS (energy levels) at specific distances from the nucleus.
Each shell has a fixed energy — electrons can jump between shells by absorbing or emitting energy.
The Modern Understanding
After Bohr's model, further work led to the modern understanding:
Protons and neutrons were later identified in the nucleus.
NEUTRON discovery (Chadwick, 1932) explained why atomic mass was not just the number of protons.
Why each model was better:
Dalton → Thomson: discovery of the electron showed atoms had internal structure.
Thomson → Rutherford: gold foil experiment showed positive charge was concentrated in a tiny nucleus, not spread throughout.
Rutherford → Bohr: observed line spectra of hydrogen showed electrons could only occupy specific energy levels.
This chain of model improvements shows the power of the scientific method — each experiment built on previous knowledge and revealed new detail about atomic structure.
⚠️ Common Mistake
In Rutherford's experiment, MOST particles passed STRAIGHT THROUGH — this is what shows atoms are mostly empty space. Only a FEW bounced back. Students often get this backwards and say most particles bounced back. The few that did bounce back were what proved the nucleus exists.
📌 Key Note
Dalton: solid sphere. Thomson: plum pudding (electrons in positive mass). Rutherford: nuclear model — most particles pass through (empty space), a few bounce back (dense positive nucleus). Bohr: electrons in fixed shells. Each model revised due to new experimental evidence.
🎯 Matching Activity — Match the Model to its Scientist and Evidence
Match each atomic model to the scientist and what evidence led to it. — drag the symbols on the right to match the component names on the left.
Dalton
Drop here
Thomson
Drop here
Rutherford
Drop here
Bohr
Drop here
Shell model — electrons orbit in fixed energy levels around the nucleus
Plum pudding — discovery of the electron showed atoms had internal structure
Nuclear model — gold foil experiment showed a tiny dense positive nucleus
Solid indivisible sphere — no subatomic particles known at this time
🎯 Test Yourself
Question 1 of 2
1. In Rutherford's gold foil experiment, most alpha particles passed straight through the foil. What does this tell us?
2. Why did the scientific community accept Rutherford's nuclear model over Thomson's plum pudding model?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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