An early model of the structure of the atom was called the plum pudding model. In this model, the atom was imagined to be a sphere of positive charge with negatively charged electrons dotted around inside it - like plums in a pudding.
Scientific models can be tested to see if they are correct by doing experiments. An experiment carried out in 1909 showed that the plum pudding model could not be correct.
A scientist called Ernest Rutherford designed an experiment to test the plum pudding model. It was carried out by his assistants Hans Geiger and Ernest Marsden.
A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. The scientists expected the alpha particles to pass straight through the foil, but something else also happened.
Some of the alpha particles emerged from the foil at different angles, and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the centre of the atom (the nucleus).
As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.