Rutherford Scattering

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.

  • Rutherford used gold because it is very malleable – so it can be stretched into very thin sheets, leading to very useful results.
  • The gold foil had to be placed in a vacuum to ensure that the atoms were only colliding with gold atoms and not anything else.

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:

A visual representation of Rutherford's gold foil experiment. On the left, a radioactive source covered with a lead block emits a beam of alpha particles towards a thin gold foil in the centre. As the alpha particles hit the foil, three behaviours are observed: Most particles continue straight, suggesting atoms are mostly open space. Some deflect slightly, indicating interactions with other charged particles within the atom. A few bounce straight back, highlighting a concentrated positive charge in the atom's centre. Insets detail these interactions with dot diagrams: the majority passing straight, some slightly deflected, and a rare few bouncing back. The path of the particles is captured on a fluorescent screen forming a circular pattern around the gold foil.

These conclusions contradicted the plum pudding model, leading to its replacement with the nuclear model.

An illustration of Rutherford's nuclear model from 1911. The atom is depicted with a central red nucleus carrying a positive charge, surrounded by yellow orbits. Blue electrons with negative charges circulate in these orbits, demonstrating the structure of an atom with electrons revolving around a densely packed nucleus. The title "Nuclear model (Rutherford, 1911)" is labelled below.

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.