Final answer:
When the isotope Bi undergoes alpha decay, it forms the isotope Tl.
Step-by-step explanation:
When the isotope Bi undergoes alpha decay, it forms the isotope Tl. Alpha decay occurs when an unstable nucleus emits an alpha particle, which is a helium-4 nucleus consisting of two protons and two neutrons. This results in the atomic number of the nucleus decreasing by 2, which changes the identity of the element. In the case of Bismuth (Bi), it undergoes alpha decay to form Thallium (Tl).