Lead is the heaviest naturally occurring element with a stable isotope.
Lead has atomic number 82 and the symbol Pb, derived from the Latin word plumbum. Several lead isotopes are stable, including lead-204, lead-206, lead-207, and lead-208. Lead-208 is also the heaviest known stable nuclide by mass number.
The neighboring element bismuth, atomic number 83, was long regarded as stable, but bismuth-209 was shown to be radioactive with an extraordinarily long half-life. That discovery makes lead the conventional answer to the stable-isotope distinction.
Lead is a dense, soft metal used historically in pipes, pigments, ammunition, radiation shielding, and batteries. Its toxicity means many older uses have been restricted or replaced. The answer concerns isotope stability, not density or atomic mass alone.