Technetium

Element 43 was one of Mendeleev's gaps, eka-manganese, and it stayed stubbornly empty because it has no stable isotope — a consequence of Mattauch's isobar rule, which leaves both odd-Z neighbours of molybdenum and ruthenium without a stable partner. Carlo Perrier and Emilio Segrè finally identified it in 1937 in a discarded molybdenum deflector foil that Ernest Lawrence had irradiated in the Berkeley cyclotron and posted to Palermo, and they named it technetium, the artificial one. The figure of 98 quoted here is the mass number of the longest-lived practically useful isotope, not a measured atomic weight. Technetium-99m, with a 6-hour half-life and a clean 140 keV gamma, is the most used radionuclide in medicine, behind tens of millions of diagnostic scans a year. Its detection in the spectra of red giants was direct proof that stars make elements.