Rutherford and the nucleus

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.

curriculum-key-fact
Alpha particles are a form of nuclear radiation with a large positive charge.

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.

Alpha radiation beamed to a thin piece of gold foil which is in a scintillation screen.

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:

  • most of the alpha particles did pass straight through the foil
  • a small number of alpha particles were deflected by large angles (> 4°) as they passed through the foil
  • a very small number of alpha particles came straight back off the foil
Alpha raditaion beamed through gold nuclei, with most passing through unaffected, some deflected at small angle and very few rays being almost reflected back.

Rutherford considered these observations and he concluded:

  • The fact that most alpha particles went straight through the foil is evidence for the atom being mostly empty space.
  • A small number of alpha particles being deflected at large angles suggested that there is a concentration of positive charge in the atom. Like charges repel, so the positive alpha particles were being repelled by positive charges.
  • The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus). The tiny number of affected particles means the chance of being on that exact collision course was very small, therefore the 'target' being aimed at had to be equally tiny.
curriculum-key-fact
Rutherford's experiment showed the existence of a nuclear atom - a small, positively-charged nucleus surrounded by empty space and then a layer of electrons to form the outside of the atom.
What happenedRutherford'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.