Atom of Ernest Rutherford, bombarded a metal foil with alpha particles to observe how they scattered. They expected all the alpha particles to pass straight through with little deflection, because Thomson's model said that the charges in the atom are so diffuse that their electric fields could not affect the alpha particles much. However, Geiger and Marsden spotted alpha particles being deflected by angles greater than 90°, which was supposed to be impossible according to Thomson's model. To explain this, Rutherford proposed that the positive charge of the atom is concentrated in a tiny nucleus at the center of the atom.
Mister Atom long, however: In "52", Captain Marvel mentioned that Mister Atom attacked the Rock of Eternity, only to fail. It is later revealed that Mister Atom is one of the villains who helped Prometheus plant explosive devices in various cities across the U.S., which leads to Mister Atom and several other villains making an attempt to flee the country. Before they can cross the border, the group is attacked by the Justice League, and Mister Atom is defeated and captured by Starman and Congorilla. Mister Atom has upper level superhuman strength and endurance. His robot body is virtually impervious to harm.