Lead-208

Lead (₈₂Pb) 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: the uranium series (or radium series), the actinium series, and the thorium series, respectively. These series represent the decay chain products of long-lived primordial U-238, U-235, and Th-232, respectively. However, each of them also occurs, to some extent, as primordial isotopes that 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).

The longest-lived radioisotopes are ²⁰⁵Pb with a half-life of ≈15.3 million years and ²⁰²Pb with a half-life of ≈53,000 years. Of naturally occurring radioisotopes, the longest half-life is 22.3 years for ²¹⁰Pb, which is useful for studying the sedimentation chronology of environmental samples on time scales shorter than 100 years.

The relative abundances of the four stable isotopes are approximately 1.5%, 24%, 22%, and 52.5%, combining to give a relative atomic mass (abundance-weighted average of the stable isotopes) of 207.2(1). Lead is the element with the heaviest stable isotope, ²⁰⁸Pb. (The more massive ²⁰⁹Bi, long considered to be stable, actually has a half-life of 1.9×10¹⁹ years). A total of 38 Pb isotopes are now known, including very unstable synthetic species.