If this were so, there would be 3 lead isotopes that would differ not in radioactive properties, but in atomic weight. The isotopes could be referred to as lead-206, lead-207, and lead-208. If we use the chemical symbol for lead (Pb), we could write the isotopes, ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb. (We read the symbol ²⁰⁶Pb as lead-206.) Atomic weight measurements made in 1914 by Soddy and others supported that theory.
All 3 lead isotopes had the same atomic number of 82. The atoms of all 3 isotopes had nuclei with an electric charge of +82 and all 3 had 82 electrons in the atom to balance that positive nuclear charge. The difference was in the mass of the nucleus only.
Isotopes of two elements.
But what of ordinary lead that existed in the rocks far removed from any radioactive substances and that had presumably been stable through all the history of earth? Its atomic weight was 207.2.
Was the stable lead that had no connection with radioactivity made up of atoms of still another isotope, one with a fractional atomic weight? Or could stable lead be made up of a mixture of isotopes, each of a different whole-number atomic weight and was the overall atomic weight a fraction only because it was an average?
It was at the moment difficult to tell in the case of lead, but an answer came in connection with another element, the rare gas neon (atomic symbol Ne), which has an atomic weight of 20.2.
Was that fractional atomic weight something that was possessed by all neon atoms without exception or was it the average of some lightweight atoms and some heavyweight ones? It would be a matter of crucial importance if isotopes of neon could be found, for neon had nothing to do with any of the radioactive series. If neon had isotopes then any element might have them.
In 1912 Thomson was working on neon. He sent a stream of cathode-ray electrons through neon gas. The electrons smashed into the neon atoms and knocked an electron off some of them. That left a neon ion carrying a single positive charge—an ion that could be written Ne⁺.
The neon ions move in the electric field as electrons do, but in the opposite direction since they have an opposite charge. In the combined presence of a magnet and of an electric field, the neon ions move in a curved path. If all the neon ions had the same mass, all would follow the same curve. If some were more massive than others, the more massive ones would curve less.