The discovery of the electron, a negatively charged subatomic particle, led to the proposal of the plum pudding model of the atom by J.J. Thomson in 1904. Thomson's experiments with cathode rays demonstrated the existence of electrons, which were then thought to be embedded like "plums" within a positively charged "pudding" or sphere, accounting for the overall neutrality of the atom. This model visualized the atom as a positively charged sphere with electrons distributed throughout it.