Scientists change a model when new evidence does not fit it. The idea of the atom has changed several times. In 1897 Thomson found the electron. His plum pudding model pictured the atom as a ball of positive charge with negative electrons embedded in it.
Later, Geiger and Marsden, working with Rutherford, fired alpha particles at thin gold foil. The plum pudding model predicted that they would all pass through with only tiny deflections. Instead, most passed straight through, a few were deflected by large angles and a very few bounced back.
Rutherford explained this with the nuclear model. The atom is mostly empty space, and its positive charge and most of its mass are concentrated in a tiny nucleus. Bohr then improved the model by placing electrons in fixed energy levels, also called shells, at set distances from the nucleus. Geiger and Marsden did the experiments and Rutherford interpreted them, so scientists worked together to develop the model.
