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.

 Exploring the Interactive

Using the PhET simulation below, we will compare the two competing atomic models:

1. Select the Plum Pudding Atom: Observe how alpha particles were expected to behave—passing straight through with minimal deflection.
2. Observe the Discrepancy: Note that this continuous pass-through is not what occurred during the actual gold foil experiment.
3. Select the Rutherford Atom: Examine the actual experimental results.
4. 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.

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.