Lead-208 is the heaviest "stable" nuclide, assuming proton decay is not possible, but it is not the only stable nuclide. Excluding isotopes like argon-36 (which

Lead-208 is the heaviest "stable" nuclide, assuming proton decay is not possible, but it is not the only stable nuclide. Excluding isotopes like argon-36 (which has an energetically permitted double electron capture to sulfur-36) and not considering possible spontaneous fissions for those A>92, there are still 146 nuclides (all of which Z<67) that are stable. Incidentally, astute readers that have picked up on the fact that Pb has Z=82 which is larger than 66 should pat themselves on the back and note that the 4 isotopes all have a yet-to-be-observed decay to mercury, with half life of >1.4×10^20 years for 204 and experimental lower bounds of >10^21 for the other 3. Theoretically, 208 takes >10^124, which means 207 is "more" stable.