The scientist who made the next major change to the atomic model was Ernest Rutherford. In 1911, Rutherford carried out an experiment to test the plum pudding model, with his two students Hans Geiger and Ernest Marsden. They directed a beam of alpha particles (helium nuclei) at really thin pieces of gold foil that were suspended in a vacuum.
This experiment was conducted to test how many alpha particles were absorbed by the foil. An alpha particle is a form of nuclear radiation that has a huge positive charge.
J.J. Thomson had proposed that the positive charge in gold atoms was spread out. Therefore, the weak spread of positive charge should not have been strong enough to have a large effect on the alpha particles. This is why Rutherford expected the alpha particles to pass through the gold foil and maybe slightly change direction.
During this experiment, he made a number of observations and came to new conclusions:
These conclusions contradicted the plum pudding model, leading to its replacement with the nuclear model.
Instead of a general positive charge, the positive charge is concentrated in the centre of the atom, in a compact nucleus. Rutherford thought that the electrons must exist in a cloud around the nucleus.