Question

19. A radioactive nucleus decays to yield a bismuth-211 nucleus and an alpha particle. What was the identity of the original nucleus? Show the nuclear equation that leads you to this answer.

Answer 1

It's hard to do subscripts and superscripts here so here is what I will do.
A is the atomic number, M is the mass number, X is the element. so
AX^M ==> 2He^4 + 83Bi^211
Now you want the subscripts to add up on both sides as well as the superscripts to add up on both sides.
A = 83 + 2 = 85 and you look on the periodic table to find the identity of X
M = 211 + 4 = ?
Now fill in the AX^N we started with.
85At^215 = 2He^4 + 83Bi^211