A scientific model changes when new experimental evidence does not fit it. 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.
In the alpha particle scattering experiment, positively charged alpha particles were fired at thin gold foil. Most went straight through, but a few were deflected through large angles and a very small number bounced back. This did not fit the plum pudding model. It showed that the mass of the atom is concentrated at the centre, in a nucleus, and that the nucleus is positively charged. This nuclear model replaced the plum pudding model, with most of the atom being empty space.
Niels Bohr then adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. His theoretical calculations agreed with experimental observations. Later experiments showed that the positive charge of a nucleus can be split into a whole number of smaller particles, each with the same charge, and these were named protons. About 20 years after the nucleus was accepted, James Chadwick provided evidence for neutrons in the nucleus.
