Thomson's plum pudding model, proposed in 1904, suggested that atoms consist of a uniform positive charge throughout the atom with electrons embedded within like "plums in plum pudding". However, Thomson's model did not explain how the positive charge was arranged within the atom.

In 1911, Ernest Rutherford performed the gold foil experiment. In this experiment, a beam of alpha particles (positively charged helium nuclei) was fired at a thin sheet of gold foil. Most of the alpha particles passed through the foil without any deflection. However, a small number of alpha particles were deflected at large angles or even bounced back.

Rutherford's experiment showed that the positive charge of the atom must be concentrated in a small, dense nucleus. This led to the development of the Rutherford model of the atom, which consists of a small, positively charged nucleus surrounded by negatively charged electrons in fixed circular orbits.