High Quality Content by WIKIPEDIA articles! Lead (Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains called the uranium series (or radium series), the actinium series, and the thorium series, respectively. These series represent the decay chain products of long-lived primoridal U-238, U-235, and Th-232, respectively. However, each of them also occurs, to some extent, as primordial isotopes which were made in supernovae, rather than radiogenically as daughter products. The fixed ratio of lead-204 to the primordial amounts of the other lead isotopes may be used as the baseline to estimate the extra amounts of radiogenic lead present in rocks as a result of decay from uranium and thorium. (See lead-lead dating and uranium-lead dating).