New experimental evidence can lead to a scientific model being changed or replaced. Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided. The discovery of the electron led to the plum pudding model, in which the atom is a ball of positive charge with negative electrons embedded in it.
The alpha particle scattering experiment gave results that the plum pudding model could not explain. These results led to the conclusion that the mass of an atom is concentrated at the centre, in a nucleus, and that the nucleus is charged. The nuclear model replaced the plum pudding model.
Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. His theoretical calculations agreed with experimental observations.
Later experiments led to the idea that the positive charge of a nucleus could be divided into a whole number of smaller particles, each with the same amount of positive charge. These particles were named protons. About 20 years after the nucleus became accepted, James Chadwick provided evidence for neutrons in the nucleus.
