Naturally occurring terbium (Tb) is composed of 1 stable isotope, 159Tb. Thirty-six radioisotopes have been characterized, with the most stable being 158Tb with a half-life of 180 years, 157Tb with a half-life of 71 years, and 160Tb with a half-life of 72.3 days. All of the remaining radioactive isotopes have half-lives that are less than 6.907 days, and the majority of these have half-lives that are less than 24 seconds. This element also has 27 meta states, with the most stable being 156m1Tb (t1/2 24.4 hours), 154m2Tb (t1/2 22.7 hours) and 154m1Tb (t1/2 9.4 hours).
The primary decay mode before the most abundant stable isotope, 159Tb, is electron capture, and the primary mode behind is beta decay. The primary decay products before 159Tb are element Gd (gadolinium) isotopes, and the primary products behind are element Dy (dysprosium) isotopes.
Relative atomic mass: 158.92535(2).