New experimental evidence may lead to a scientific model being changed or replaced. Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided. The discovery of the electron led to the plum pudding model. This suggested that 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 a thin sheet of gold foil. Most went straight through, which showed that most of an atom is empty space. A small number were deflected, and a few bounced back. This led to the conclusion that the mass of an atom is concentrated at the centre, in a small 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 calculations agreed with experimental observations. Later experiments led to the idea that the positive charge of any nucleus could be divided into a whole number of smaller particles, each with the same amount of charge. These were named protons. About 20 years after the nucleus became an accepted idea, the experimental work of James Chadwick provided evidence for neutrons in the nucleus.
