Stefanie Horovitz (1887-1942) made significant contributions to the discovery of isotopes. English radiochemist Frederick Soddy (1877-1956) had proposed that radioactive elements disintegrate step-wise to form lead. However, based on Soddy’s Group Displacement law, which describes which elements and isotopes are created during a particular type of radioactive decay (alpha or beta), Soddy expected the atomic weight for the lead produced by uranium decay to be 206.5. But the atomic weight for common lead was then measured as 207.1.
So, Soddy proposed that these were forms of the same element with different masses and should be called isotopes. But at this time, the concept of atoms being identical in everything but weight was not generally accepted.
In order to support his idea, Soddy asked Horovitz to show that lead from radioactive sources had a different atomic weight than that of common lead. To do so, she first had to separate out very pure samples of lead from residues. Then she took measurements to determine the atomic weights, each made to the nearest 100th of a gram.
Horovitz found the atomic weight of the lead from the disintegration of uranium and radium to be 206.736 and the atomic weight for common lead to be 207.190. The analysis of other samples from different mines gave weight values as low as 206.046. This was the first concrete evidence for the concepts of isotopes.
In addition to these exceptionally precise measurements, Horovitz also helped to disprove the existence of the radioactive element ionium. In 1907, American pioneer of radiochemistry Bertram Boltwood (1870-1927) announced that he had discovered ionium. His postulation was accepted among most radioactivity researchers. However, between 1914 and 1916, Horovitz determined the precise atomic weight of ionium, showing that it had the same spectroscopic and chemical properties as thorium and only differed in atomic weight. Thus, it was not a new element but rather an isotope of thorium, thorium-230.