The Gold Foil Experiment — Rutherford's Nucleus

The apparatus, live

Fire alpha particles at the foil

This is a schematic 2D model of the real setup: an alpha source, a lead collimator to narrow the beam, a thin gold foil, and a scintillation screen that flashes wherever a particle lands. Press start and watch where the particles end up.

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Beam rate Particles glow white-hot when passing close enough to a nucleus to deflect. Deflection frequency is exaggerated for visibility — see teaching notes below.

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Particles fired

Passed straight through

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Deflected

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Bounced back (>90°)

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Two competing pictures of the atom

What scientists expected vs. what they found

Before this experiment, J.J. Thomson's "plum pudding" model was the accepted picture of the atom. The scattering results were only explainable by replacing it with something very different.

Prevailing model, pre-1911

Plum pudding model

Positive charge is spread evenly through the whole atom, like a diffuse "pudding," with negative electrons studded through it.

Predicted: alpha particles should pass through with only small, gentle deflections — the positive charge is never concentrated enough anywhere to repel them strongly.

Rutherford's model, 1911

Nuclear model

Almost all of the atom's mass and all of its positive charge sit in a tiny central nucleus. The rest of the atom — where the electrons are — is mostly empty space.

Predicted: most alpha particles sail through the empty space untouched; a rare few pass close enough to the concentrated charge to be deflected sharply, even straight back.

From data to theory

Three observations, three conclusions

Rutherford didn't run the experiment himself — Geiger and Marsden counted the scintillation flashes, angle by angle, over months. It was Rutherford who realised what the pattern of results demanded.

Observed

The overwhelming majority of alpha particles passed straight through the foil, or deflected by only a degree or two.

Implies

An atom is mostly empty space. There is nothing solid enough in most of its volume to stop or deflect a fast, heavy alpha particle.

Observed

A small number of particles were deflected through large angles — far more than the plum pudding model could account for.

Implies

Somewhere inside the atom there is a concentration of positive charge strong enough to repel a positively charged alpha particle noticeably.

Observed

About 1 in every 8000 particles bounced back at an angle greater than 90°, occasionally almost straight back toward the source.

Implies

That charge — and nearly all of the atom's mass — must be packed into a nucleus thousands of times smaller than the atom itself.

Rutherford later said the backscattering result was almost as unbelievable as firing a heavy artillery shell at a sheet of tissue paper and having it bounce straight back at you.

For the classroom

Teaching notes

Simplifications in this simulation — worth flagging to students

Deflection rate is wildly exaggerated. In reality only about 1 in 8000 alpha particles was deflected by more than 90°; here it's tuned much higher so a class can actually see it happen within a minute.
Scale is not accurate. A real nucleus is roughly 100,000 times smaller than the atom it sits inside — if the nucleus were the size of a marble, the atom would be about the size of a sports stadium. The gold nuclei here are drawn far larger than that ratio so they're visible at all.
Each particle here interacts with one nucleus. A real foil is several hundred atoms thick, so a particle could in principle pass close to more than one nucleus — this model keeps it to a single close encounter for clarity.
The model is 2D. The real experiment, and the real atom, scatter particles in three dimensions; the screen was a full ring around the foil, not a flat arc.