Nuclear model

Rutherford used the evidence to develop the nuclear model: almost all the atom’s mass and its positive charge are concentrated in a tiny central nucleus, while electrons occupy the space around it. Since most alpha particles encountered empty space, only a very small proportion came close enough to the nucleus to be strongly deflected.

Niels Bohr refined the nuclear model by proposing that electrons occupy particular shells around the nucleus, rather than being able to exist at any distance from it. Later, James Chadwick discovered the neutron, showing that the nucleus contains neutral particles as well as positively charged protons. Each change kept what earlier evidence could explain while replacing a part of the model that new evidence showed was incomplete.