For the answer to the question above,
Thallium-206 decays by beta emission to make the next element to the right in the periodic table, which is lead-206.
The decay equation:
81Tl206 --> -1e0 + 82Pb206
A neutron in Tl-206 decays, producing a proton and a beta particle. Since a proton is produced the atomic number increases by 1, and since it "swapped" a neutron for a proton, the mass number stays the same.
