September 22nd marks the passing of Frederick Soddy. Soddy was an English chemist who investigated the assortment of newly discovered elements that were decayed from radium, thorium and uranium. At this time, many of these elements had names like mesothorium, Radium A, B, C, D, E, F and Uranium X, each with a different half-life. Chemists were trying to separate elements like radiothorium from thorium but could not accomplish the task. Soddy theorized they were all the same elements and could not be separated chemically. They had slightly different atomic masses, but were the same elements. He gave the name 'isotope' to the atoms in this elemental mixture. He cited radium D and thorium C were actually two different isotopes of lead and acted the same as lead chemically.
He couldn't explain his theory with the current science. When the neutron was discovered by James Chadwick, Soddy's isotope theories suddenly made more sense. He also described the mechanics of radioactive decay. Alpha decay was the emission of a helium nucleus, 2 protons and 2 neutrons that lowered the atomic number by 2 and the mass by 4. Beta decay involved the emission of a beta particle (electron) that raised the atomic number by 1.
Soddy was awarded the 1921 Nobel Prize in Chemistry for this work and his theory. Find out what else occurred on this day in science history.