Which element is the lightest element with no stable isotopes?

Technetium is the lightest element with no stable isotopes; every technetium isotope is radioactive.

Technetium has atomic number 43 and lies between molybdenum and ruthenium in the periodic table. Its lack of a stable isotope results from nuclear-structure effects: for each relevant mass number, a neighboring element has a more stable nuclear arrangement. The only lighter element with the same broad distinction does not exist; the next such element is promethium, atomic number 61.

Dmitri Mendeleev predicted a gap for element 43, but technetium was not produced and identified until 1937, when researchers bombarded molybdenum. Its name comes from the Greek technetos, meaning “artificial,” because it was the first predominantly artificial element discovered.

Technetium is now especially important in medicine. Technetium-99m, a short-lived nuclear isomer, emits gamma rays useful for diagnostic imaging. Technetium-99, by contrast, has a half-life of about 211,100 years and is a significant nuclear-fission product.