Frederick Soddy
There was, in short, a “radioactive series” beginning with uranium (atomic number 92) and ending with lead (atomic number 82). The same was true of thorium (atomic number 90), which began a series that also ended with lead. Still a third element, actinium (atomic number 89) was, at that time, the first known member of a series that also ended in lead.
The various atoms formed in these three radioactive series were not all different in every way. When the uranium atom gives off an alpha particle, it forms an atom originally called “uranium X₁”. On close investigation, it turned out that this uranium X₁ had the chemical properties of thorium. Uranium X₁, had, however, radioactive properties different from ordinary thorium.
Uranium X₁ broke down so rapidly, giving off beta particles as it did so, that half of any given quantity would have broken down in 24 days. Another way of saying this (which was introduced by Rutherford) was that the “half-life” of uranium X₁, is 24 days. Ordinary thorium, however, gives off alpha particles, not beta particles, and does so at such a slow rate, that its half-life is 14 billion years!
Uranium X₁, and ordinary thorium were in the same place in the list of elements by chemical standards, and yet there was clearly something different about the two.
Here is another case. In 1913 the British chemist Alexander Fleck (1889- ) studied “radium B” and “radium D”, the names given to two different kinds of atoms in the uranium radioactive series. He also studied “thorium B” in the thorium radioactive series and “actinium B” in the actinium radioactive series. All four are chemically the same as ordinary lead; all four are in the same place in the list of elements. Yet each is different from the radioactive standpoint. Though all give off beta particles, radium B has a half-life of 27 minutes, radium D one of 19 years, thorium B one of 11 hours, and actinium B one of 36 minutes.
In 1913 Soddy called atoms that were in the same place in the list of elements, but which had different radioactive properties, “isotopes”, from Greek words meaning “same place”.
At first, it seemed that the only difference between isotopes might be in their radioactive properties and that only radioactive atoms were involved. Quickly that proved not to be so.
It proved that it was possible to have several forms of the same element that were all different even though none of them were radioactive. The uranium series, the thorium series, and the actinium series all ended in lead. In each case the lead formed was stable (not radioactive). Were the lead atoms identical in every case? Soddy had worked out the way in which atomic weights altered every time an alpha particle or a beta particle was given off by an atom. Working through the three radioactive series he decided that the lead atoms had different atomic weights in each case.
The uranium series ought to end with lead atoms that had an atomic weight of 206. The thorium series ought to end in lead atoms with an atomic weight of 208 and the actinium series in lead atoms with an atomic weight of 207.