By now fifty radioactive elements had been reported, but the periodic table only had space for about ten to twelve. Soddy suggested the radioactive displacement law: an atom that gives off an alpha or beta particle must change into a different substance. Isotopes are forms of the same element with equal nuclear charge but different masses, distinguished by their radioactive properties. Thus multiple substances could share one spot in the periodic table.
A visual chronology of Asimov's ~1500 scientific inventions and discoveries.
