Mononuclidic elements in the context of Stable nuclide

Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, Th, is relatively stable, with a half-life of 1.40×10 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty-one radioisotopes have been characterized, with the most stable being Th, Th with a half-life of 75,400 years, Th with a half-life of 7,916 years, and Th with a half-life of 1.91 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be 8.355733554021(8) eV It has been proposed to perform laser spectroscopy of the Th nucleus and use the low-energy transition for the development of a nuclear clock of extremely high accuracy.