Learn the idea: a short explanation, then a check.
J.J. Thomson’s discovery of the electron led him to propose the plum pudding model of the atom, which pictured negatively charged electrons embedded within a uniform sphere of positive charge.
To test this hypothesis, Ernest Rutherford—along with Hans Geiger and Ernest Marsden—designed the famous gold foil experiment. Their unexpected observations of alpha particle deflection radically transformed our understanding of atomic structure, disproving the plum pudding model and revealing the existence of a dense, positively charged nucleus.
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Worked examples first, then problems to try, then worksheets.
Check Your Understanding: Rutherford's Scattering Experiment
1. In Rutherford's experiment, a beam of alpha particles was fired at a very thin gold foil. What was observed?
Most alpha particles passed straight through, or were deflected by only a small angle. A small number were deflected through large angles, and a very few bounced back almost the way they came. The last result was a complete surprise.
The surprising result is that a few alpha particles were deflected through large angles, and a very few bounced almost straight back. Most passed straight through, but not all of them, and the foil did not absorb them. A model with no dense centre would predict only small deflections.
2. What did the few alpha particles that bounced almost straight back tell Rutherford?
To bounce an alpha particle straight back, a strong force from a very concentrated positive charge is needed. Rutherford concluded that all the positive charge, and most of the mass, is packed into a tiny nucleus at the centre of the atom. This repels the positive alpha particles.
Electrons are far too light to turn an alpha particle back, and evenly spread positive charge would give only small deflections. The "empty space" idea comes from the particles that went straight through. The back-scattered ones showed a tiny, dense, positive nucleus.
3. Why did most of the alpha particles pass straight through the gold foil?
The nucleus is about 10,000 times smaller than the atom, so most alpha particles never came close enough to a nucleus to be deflected much. They passed through the empty space between the nuclei, which is where the electrons are found.
The foil was a few thousand atoms thick, and alpha particles are positively charged. Electrons are far too light to guide them. Most went straight through because the atom is mostly empty space, with a tiny nucleus at its centre.
4. How did Rutherford's result change the "plum pudding" model of the atom?
In the plum pudding model, the positive charge is spread through the whole atom with the electrons embedded in it, which cannot deflect alpha particles by large angles. Rutherford's nuclear model puts all the positive charge in a tiny nucleus, with the electrons around it.
Electrons are negative, and neutrons were not discovered until later (by Chadwick). The experiment showed that atoms do have structure, which is the opposite of "cannot be divided". Its big effect was to replace the plum pudding with a small, dense nucleus surrounded by electrons.
5. What was a key limitation of Rutherford's nuclear model of the atom?
An electron orbiting the nucleus is accelerating, so classical physics says it should radiate energy and spiral into the nucleus in a tiny fraction of a second. Atoms are stable, so something was missing. The model also could not explain the line spectra of atoms, or what the nucleus is made of. These problems led to the Bohr model.
The model explained the scattering very well, and it said that atoms are mostly empty space. Its weakness was the electrons: classical physics says an orbiting electron should radiate energy and spiral into the nucleus, so atoms should collapse. It also could not explain line spectra. These problems led to the Bohr model.
🕹️Explore
Test your predictions in a simulation.
Exploring the Interactive
Using the PhET simulation below, we will compare the two competing atomic models:
Select the Plum Pudding Atom: Observe how alpha particles were expected to behave—passing straight through with minimal deflection.
Observe the Discrepancy: Note that this continuous pass-through is not what occurred during the actual gold foil experiment.
Select the Rutherford Atom: Examine the actual experimental results.
Inspect the Nucleus: Click on the nucleus to see how incoming alpha particles approach, strike, or pass near the dense positive core, experiencing strong electrostatic repulsion and deflection.
In this video, I use a 3D-printed model to demonstrate how Rutherford's scattering experiment works, allowing you to participate along the way. I also discuss how science relies on building models to explain experimental observations and accurately predict future outcomes.
If you would like to 3D-print your own version of this model, you can download the design files here. It was created by S'Cool LAB, a physics education research project based at CERN.
It is important to note that while Rutherford's nuclear model successfully explained the results of the gold foil experiment, it was not complete.
As mentioned in the video, classical physics—specifically Maxwell’s theory of electromagnetism—dictated that accelerating charged particles (such as orbiting electrons) must continuously radiate energy. As a result, electrons should rapidly spiral inward and collapse into the nucleus, making stable atoms impossible under classical mechanics.
Resolving this instability required a quantum approach, which we explore in our lesson on Bohr's Model.
The next lesson covers the discovery of the neutron.