Geiger-Marsden experiment

Dalton's model

John Dalton published his ideas about atoms in 1803. He thought that all matter is made of atoms, which he imagined as tiny solid balls. Dalton's model included these ideas:

  • atoms cannot be broken down into anything simpler
  • the atoms of a given element are identical to each other
  • the atoms of different elements are different from one another

The Dalton model has changed over time because of the work of other scientists.

Experiments by Geiger and Marsden

In the 100 years after Dalton's model of the atom, a number of chemists tried to work out the structure of the atom. In 1904, J J Thomson proposed a new model for the atom. This model showed a positively charged sphere, with negative electrons embedded - like currants in a Christmas pudding. This was called the plum pudding model.

Image of a plum pudding model, with a large blue circle with a positive symbol behind six red smaller circles with negative symbols.The plum pudding model

This model was tested by the scientists Geiger and Marsden in 1909. They set up a very thin layer of gold foil and fired alpha particles - radioactive particles with a positive charge - at the gold. The scientists expected most of the alpha particles to travel through the gold foil and to be deflected slightly as they passed close to an electron 'currant'.

Alpha particles travel from the alpha source and bounce off gold atoms.The alpha particle scattering experiment

In fact, the alpha particles were deflected much more than expected - some of them seemed to bounce almost straight back. The scientist Ernest Rutherford realised that the only way to explain results from experiments by Geiger and Marsden was with a new model of the atom.

Rutherford model of the atom

When Rutherford saw the results of the experiment by Geiger and Marsden, he said:

quote
It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.

Rutherford used the results of this experiment to develop a new model for the atom. This model proposed a central nucleus with a positive charge. It was this positively charged nucleus that was responsible for the strong backward deflection of the positively charged alpha particles.

The model also proposed that negatively charged electrons surrounded this nucleus. However, as most of the alpha particles passed through the gold foil with no deflection at all, Rutherford realised that most of the atom was empty space. So his model placed the electrons at some distance from the nucleus.