Although thorium (Th), with atomic number 90, has 6 naturally occurring isotopes, none of these isotopes are stable; however, one isotope, Th, is relatively stable, with a half-life of 14.05 billion 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. As such, thorium is considered to be mononuclidic. It has a characteristic terrestrial isotopic composition and thus an atomic mass can be given.
Relative atomic mass: 232.03806(2).