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Theories of the atom

The idea of atoms as small particles was put forward by the Greeks 2000 years ago, but the structure of the inside of the atom was not understood until the beginning of the twentieth century!

The English scientist Thomson suggested that the atom, which is a neutral particle, was made of positive charge with 'lumps' of negative charge inset in it - rather like the plums in a pudding. For this reason it was known as the Plum Pudding theory of the atom. (Figure 1)

However, there were problems with this model but nobody could think of a better one. Then in 1911 Rutherford, Geiger and Marsden were studying the passage of alpha particles through thin pieces of gold foil. As they expected, some of the alpha particles were able to pass through the film. They then noticed something that they did not expect.

Some of the alpha particles were being deflected from their original path and more surprising still about 1 in 8000 were actually knocked backwards! You must remember that although alpha particles are very small they are travelling at a tenth the speed of light.


Rutherford explained it this way. He knew that the alpha particles carried a positive charge so he said that the positive charge of the atom was concentrated in one place that he called the nucleus, and that the negatively charged particles, the electrons, were in orbit around the nucleus.

The atom is mostly empty space! Think of a small marble with a cloud of gnats round it in a sports hall. That is like the nucleus, electrons and the volume of the whole atom.

Figure 3 shows the alpha particle scattering explanation that led to the modern idea of the structure of the atom.

An atom consists of a heavy central nucleus which is positively charged surrounded by a cloud of negatively charged electrons

Alpha particle scattering animation

For the schoolphysics animation of alpha particle scattering please click the symbol:

Nuclear reactions

Hundreds of years ago the alchemists thought that they could turn one metal into another -particularly lead into gold. They never managed it!

However now, using nuclear physics, we can change one element into another by adding particles to its nucleus. It is a very slow process - it can be done only one atom at a time. Remember that a centimetre cube of lead would contain many millions of millions of atoms. One reaction is shown below, alpha particles are fired at nitrogen nuclei to change them into protons and oxygen. You should look at the Elements file for details of elements.

A similar nuclear reaction was used by Chadwick in 1932 when he discovered the neutron.
© Keith Gibbs 2011