untritium

Untritium, also called element 19 in the periodic table, is a theoretical isotope that was postulated in 1937 by a team of nuclear physicists in the United Kingdom. According to the original proposals, untritium would possess a nuclear spin of 1/2 and a half‐life of approximately 25 hours, emitting beta particles of 1.5 MeV during decay. The predicted atomic mass of untritium is 19.01 u, placing it between lithium and sodium in the periodic table.

The element was never synthesized in a laboratory setting. Early proposals relied on the spallation of heavier nuclei by high-energy photons, but subsequent experiments using cyclotrons and synchrotrons failed to produce any detectable signature. Modern theoretical models suggest that the binding energy of the proposed configuration would be too low to maintain a stable neutron–proton ratio, leading to a rapid decay that could not be isolated.

Despite the lack of experimental confirmation, untritium has appeared in speculative literature as a potential energy source, owing to its high beta‑emission rate. The isotope was evaluated in the early 1960s as a candidate for therapeutic radiation applications, but safety assessments concluded that the short half-life and intense decay made it impractical. The concept of untritium remains primarily of academic interest, illustrating the challenges of predicting stable isotopes near the neutron drip line.