Bismuth-211 decays by emitting an alpha particle.

Bismuth has an atomic number of 83 which means that there are 83 protons and that the difference between the mass number and the atomic number which is 128 represents its number of neutrons. For emitting an alpha particle, the atomic number is reduced by 2 and the mass number is reduced by 4. Therefore, the atomic with an atomic number of 81 is Thallium (Tl). The answer is therefore,

Tl-207