The Roach in the Pudding Few scientists were happy with the plum pudding model. Why not? If the negative charge is carried by a particle, the positive charge should too. Ernest Rutherford proposed to test the plum pudding model by shooting positively charged alpha particles at an atom.

Rutherford’s Hypothesis The plum pudding model says the atom is a cloud or soup of positive charge. What would happen if you shot an α particle at a much larger atom? Rutherford thought the positive charge must be carried by positively charge particles that would float in the atom in the same way as electrons do (more plums in the pudding). What would you expect to happen if you shot α particles at that?

The Gold Foil Experiment Rutherford needed to make sure he would hit an atom (it’s a very small target), so he needed lots of atoms very close together (something very dense. Rutherford also needed to make sure he didn’t hit too many atoms, so he needed something very thin. He selected 60nm thick, gold foil. Finally, Rutherford needed to know where the α’s went. A screen of zinc sulfide was was set up around the foil. The zinc sulfide would fluoresce wherever an α hit it.

The Predictions Rutherford expected to see either no scattering (all the α’s would go straight through) or lots of scattering (most of the α’s would hit positive particles). No scattering would mean there was nothing there for the α’s to hit, as predicted by Thompson. Lots of scattering would mean there were positive particles for the α’s to hit, refuting the plum pudding model.

The Results Most of the α’s did go straight through. But some were deflected. Most surprisingly, a very few particles bounced back toward the source. This fit neither version of the haystack analogy.

The Conclusion The large number of α’s passing through meant that the atom had to be mostly empty space. But the deflections meant there had to be something there. Analyzing the deflections and reflections showed that the positive charges had to be in one tiny piece that contained almost all the atom’s mass.

The Rutherford Model Rutherford concluded that atoms must contain a tiny, dense, positively charged “nucleus”. The electrons orbit around the nucleus in much the way planets orbit the sun.