Isotopes of thallium

Thallium (Tl) has 37 isotopes with atomic masses that range from 176 to 212. ²⁰³Tl and ²⁰⁵Tl are the only stable isotopes and ²⁰⁴Tl is the most stable radioisotope with a half-life of 3.78 years. ²⁰⁷Tl, with a half-life of 4.77 minutes, has the longest half-life of naturally occurring radioisotopes.

Thallium-202 (half-life 12.23 days) can be made in a cyclotron while thallium-204 (half-life 3.78 years) is made by the neutron activation of stable thallium in a nuclear reactor.

In the fully ionized state, the isotope ²⁰⁵Tl becomes beta-radioactive, decaying to ²⁰⁵Pb, but ²⁰³Tl remains stable.

Relative atomic mass: 204.3833(2).