Thallium-201

Main isotopes of thallium

Iso­tope Decay abun­dance half-life (t1/2) mode pro­duct 203Tl 29.5% stable 204Tl syn 3.78 y β− 204Pb ε 204Hg 205Tl 70.5% stable
Standard atomic weight (Ar, standard)	[204.382, 204.385] Conventional: 204.38

Thallium (₈₁Tl) 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.

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