Uranium/thorium decay series transformations

Very soon after the radioactivity of thorium and uranium had been discovered it was found that pure samples of both of these elements were only very weakly radioactive. However, such pure samples became more and more radioactive with time until they reached a steady level identical to that in the original samples before pmification. This suggested that the uranium or thorium atoms were transforming or decaying into other radioactive daughter elements and that hitherto undiscovered series of such elements might exist. The search for the radioactive products of uranium by Marie and Pierre Curie led to the characterisation of two new elements, which were named polonium, Po, and radium, Ra. Both elements are far more radioactive than uranium and decay so rapidly that no ore deposits are formed. They exist only because they are formed constantly from naturally occurring uranium. 

A radionuclide, upon undergoing disintegration of a particular type, yields a specified nuclide. The original radionuclide is called the parent and the decay product is called the daughter. The daughter may also be a radionuclide. A succession of nuclides, each of which transforms by radioactive disintegration into the next until a stable nuclide results, is called a radioactive series. Examples of such series are the uranium series and the thorium series. 

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