Gold Foil Experiment

After his discovery of electrons in 1897, J.J Thomson proposed the “raisin-cake” or “plum pudding” model of the atom. In this model, the atom was thought to have consisted of a spherical cloud of positive charge, with the small electrons dispersed throughout the positive charge. ^[1] However, Rutherford’s work would soon prove this model incorrect.

In 1909, Rutherford, along with Hans Geiger and Ernest Marsden, developed a now famous experiment that allowed them to successfully deduce the structure of an atom. This experiment is referred to as either the Geiger-Marsden experiment or the Rutherford gold foil experiment ^[2]. In this experiment, alpha particles were shot at a very thin metal foil. If the particles passed through the foil, they struck a screen, which caused flashes of light. The scientists found that the particles that made it through the foil had a large angle of deflection after they passed through the foil. ^[3]

If Thomson’s model of the atom was correct, there should have been little to no deflection of the alpha particles. In Thomson’s model, the atom would have a near neutral charge overall, since the forces of the negative charges would almost cancel the spread out positive charge. Therefore, the alpha particles, which are positive, would not have been deflected by the atoms in the foil. ^[4]

However, since the alpha particles had been deflected at very large angles, Rutherford deduced that the atom must have a small, dense, positive nucleus. The electrons would orbit around the nucleus, but the majority of the atom would be empty space. In this model, one would expect some of the alpha particles to deflect at large angles, as the positive charge of the nucleus would repel the positive charge of the alpha particle. ^[5]

Plum Pudding Model and Planetary Model (proposed by Rutherford) and their expected results
"Geiger-Marsden Experiment", from Wikipedia. http://en.wikipedia.org/wiki/Geiger–Marsden_experiment

This experiment, as well as other experiments conducted by other scientists later on, allowed us to develop our current model of an atom.

[1] Knight, R.D. Physics for Scientists and Engineers a Strategic Approach.
[2] http://en.wikipedia.org/wiki/Geiger–Marsden_experiment
[3] Knight, R.D. Physics for Scientists and Engineers a Strategic Approach.
[4] Knight, R.D. Physics for Scientists and Engineers a Strategic Approach.
[5] Knight, R.D. Physics for Scientists and Engineers a Strategic Approach.