This episode introduces isotopes, first understood by Frederick Soddy, while studying decays of radioactive elements. Then we look at half-lives of elements, first calculated by Ernest Rutherford. This led to the first reasonable age of the Earth, calculated by Bertram Boltwood. Soddy and Kasimir Fajans independently figure out what happens to isotopes vis-a-vis the periodic table. Stefanie Horovitz first proves the existence of isotopes after tedious lab work to isolate two forms of lead. Soon after, J.J. Thompson builds a crude mass spectrometer and distinguishes two forms of neon. We discuss isotopes of uranium and hydrogen.Support the show Support my podcast at https://www.patreon.com/thehistoryofchemistry Tell me how your life relates to chemistry! E-mail me at steve@historyofchem.com Get my book, O Mg! How Chemistry Came to Be, from World Scientific Publishing, https://www.worldscientific.com/worldscibooks/10.1142/12670#t=aboutBook