Scientists use models to explain what atoms are like, and a model is changed when new evidence does not fit it. In the early 1800s Dalton pictured an atom as a tiny solid sphere that cannot be divided, with a different type of sphere for each element. In 1897 Thomson discovered the electron, which showed that atoms contain smaller particles. His plum pudding model described an atom as a ball of positive charge with negative electrons embedded in it.
Geiger and Marsden, working with Rutherford, fired alpha particles at a very thin sheet of gold foil. The plum pudding model predicted that the alpha particles would pass straight through with only tiny deflections. Instead most alpha particles went straight through, a few were deflected through large angles and a very few bounced almost straight back.
Rutherford explained this with the nuclear model. Most of an atom is empty space, which is why most alpha particles passed through. The positive charge and most of the mass are concentrated in a tiny nucleus at the centre, which repels and deflects the positive alpha particles. The plum pudding model could not explain this, so it was replaced. Bohr then improved the nuclear model by showing that electrons orbit the nucleus in fixed shells (energy levels) at set distances.
