Polonium the Curies while they were investigating the cause of pitchblende radioactivity. Pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than the uranium and thorium combined. This spurred the Curies to search for additional radioactive elements. They first separated out polonium from pitchblende in July 1898, and five months later, also isolated radium. German scientist Willy Marckwald successfully isolated 3 milligrams of polonium in 1902, though at the time he believed it was a new element, which he dubbed "radio-tellurium", and it was not until 1905 that it was demonstrated to be the same as polonium.
Stephanie Horovitz radiochemistry. It predicted that lead resulting from the radioactive decay of uranium or thorium would have different atomic weights than typical lead. Early experimental data was not considered authoritative by analytical chemists. Hönigschmid had studied under leading expert Theodore Williams Richards at Harvard and his work in determining precise atomic weights was well respected. Horovitz isolated lead from uranium-rich pitchblende samples from the St. Joachmistal mine. Her gravimetric analysis to the thousandth of a gram proved that lead created from the radioactive decay of uranium had a lower atomic weight than typical lead. This was the first widely-accepted experimental proof