Before the late 1800s, atoms were thought to be TINY SOLID SPHERES that could not be divided — the smallest possible particles of matter.
This model had no internal structure — atoms were indivisible 'billiard balls'.
This model was replaced when ELECTRONS were discovered.
DISCOVERY OF THE ELECTRON (J.J. Thomson, 1897):
Thomson used cathode rays — beams of particles deflected by electric and magnetic fields.
The particles (electrons) were the same regardless of the cathode material → electrons are part of all atoms.
Electrons have negative charge and very small mass.
This proved atoms CAN be divided — they have internal structure.
The solid sphere model was abandoned.
The Plum Pudding Model
After Thomson's discovery, he proposed the PLUM PUDDING MODEL:
The atom is a BALL OF POSITIVE CHARGE (the 'pudding').
NEGATIVE ELECTRONS are embedded within it (the 'plums').
The overall charge is neutral.
This seemed reasonable — it explained that atoms have electrons but are overall neutral.
However, this model was soon disproved by an important experiment.
RUTHERFORD'S ALPHA PARTICLE SCATTERING EXPERIMENT (1909, Geiger and Marsden):
Alpha particles fired at a very thin gold foil.
Expected result (plum pudding): all alpha particles would pass straight through or deflect only slightly (spread-out positive charge).
Actual result: MOST passed straight through; a SMALL NUMBER deflected through large angles; a VERY FEW bounced back almost 180°.
Rutherford's famous quote: 'It was as if you fired a 15-inch shell at tissue paper and it came back and hit you.'
The Nuclear Model
The scattering results led to the NUCLEAR MODEL (Rutherford, 1911):
Conclusions from the experiment:
MOST particles passed straight through → atom is MOSTLY EMPTY SPACE.
SOME deflected through large angles → there is a concentrated region of POSITIVE CHARGE.
VERY FEW bounced back → the positive charge is very SMALL and very DENSE.
The NUCLEAR MODEL:
Small, dense, positively charged NUCLEUS at the centre.
Electrons orbit the nucleus at relatively large distances.
Most of the atom is empty space.
FURTHER DEVELOPMENTS:
Bohr (1913): electrons orbit in fixed shells (energy levels) — explains why atoms don't collapse and why emission spectra have discrete lines.
Chadwick (1932): discovered the NEUTRON — explained why nuclei are heavier than just protons alone.
SCIENCE PRINCIPLE:
New experimental evidence leads to models being changed or replaced.
The atomic model changed as new evidence emerged — showing how science is self-correcting.
⚠️ Common Mistake
The alpha scattering experiment DISPROVED the plum pudding model — it did NOT confirm it. The fact that MOST particles passed straight through shows atoms are mostly empty space. The FEW that bounced back showed the nucleus is small and dense.
📌 Key Note
Solid sphere → plum pudding (Thomson, electrons discovered) → nuclear model (Rutherford, alpha scattering). Alpha scattering: most pass through (empty space), few bounce back (small dense positive nucleus). Bohr: electrons in fixed shells. Chadwick: neutron discovered 1932.
🎯 Matching Activity — Atomic Model Timeline
Match each model or discovery to the evidence that led to it. — drag the symbols on the right to match the component names on the left.
Solid sphere model abandoned
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Plum pudding model proposed
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Plum pudding model disproved
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Nuclear model accepted
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Electrons in fixed shells
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Alpha particle scattering — few particles bounced back, showing small dense nucleus
Bohr model — explains discrete emission spectra and stable electron orbits
Electrons discovered — atom must be positive sphere with embedded electrons
Discovery of the electron — atoms have internal structure, can be divided
Most particles pass through (empty space) + few bounce back (small dense positive nucleus)
🎯 Test Yourself
Question 1 of 2
1. In Rutherford's alpha scattering experiment, most alpha particles passed straight through the gold foil. What does this tell us?
2. Why did the results of alpha scattering disprove the plum pudding model?
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