In 1905, Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum.
The vacuum is important because any deflection of the alpha particles would only be because of collisions with the gold foil and not due to deflections off anything else.
Gold was used because it was the only metal that could be rolled out to be very, very thin without cracking.
Since the gold foil was very thin, it was thought that the alpha particles could pass straight through it, or possibly puncture the foil. The scientists were very surprised when other things happened:
Rutherford considered these observations and he concluded:
|What happened||Rutherford's conclusions|
|Most of the alpha particles did pass straight through the foil.||The atom being mostly empty space.|
|A small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil.||There is a concentration of positive charge in the atom. Like charges repel, so the positive alpha particles were being repelled by positive charges.|
|A very small number of alpha particles came straight back off the foil.||The positive charge and mass are concentrated in a tiny volume in the atom (the nucleus) - this means the chance of being on that exact collision course was very small, and so only a very small number of alpha particles would be seen to bounce straight back.|
The discovery of the make-up of the nucleus (protons and neutrons) came much later, and was not made by Rutherford. The nucleus was calculated to be about 1/10,000th the size of the atom.
If the plum pudding model was correct, there would have been very few alpha particles making it through the foil and the majority of alpha particles would have been deflected at large angles.