Bismuth-209 has a half-life of 19 quintillion years, which is over a billion times longer than the age of the universe.

For decades, bismuth-209 was listed in periodic tables as the heaviest stable isotope. This changed in 2003 when a team led by Pierre de Marcillac at the Institut d'Astrophysique Spatiale in Orsay, France, published their findings in the journal Nature. They used high-sensitivity scintillating bolometers cooled to near absolute zero to detect the alpha decay of the element.The researchers measured the half-life of bismuth-209 to be approximately 1.9 x 10^19 years. To put this in perspective, the universe is estimated to be about 13.8 billion years old. This means the half-life of this isotope is more than a billion times longer than the current age of the entire universe.Bismuth-209 decays into thallium-205 through the emission of an alpha particle. Because the decay rate is so incredibly low, the radioactivity is negligible and poses no biological risk to humans. It is so stable that if you had 100 grams of bismuth at the beginning of the universe, nearly every single gram would still be there today.This discovery shifted scientific understanding of the periodic table, making lead the heaviest element with stable isotopes. The experiment required extreme shielding from cosmic rays and terrestrial radiation to ensure the tiny signal from the bismuth decay could be isolated. This finding remains one of the most significant measurements of rare nuclear decay in modern physics.