AQA A-Level Physics/Evidence for the nucleus
In 1897, J.J Thompson discovered the electron and effectively suggested that the atom is divisible. He suggested the 'plum-pudding' model,in which the main body of the atom is a distribution of positive charge, and the electrons are embedded in this, held by electrostatic forces. The whole atom is neutral, and ions form when electrons are added or removed.
Radioactivity had already been discovered, and Ernest Rutherford used alpha particles as probes to investigate the atom. Two of his students, Geiger and Marsden, fired a thin parallel beam of alpha particles at a thin piece of gold foil.