Magnesium (Z = 12) has isotopes that range from Mg–20
to Mg–31. Only Mg–24, Mg–25, and Mg–26 are not
radioactive. What mode of radioactive decay would
convert Mg–20, Mg–21, Mg–22, and Mg–23 into stable
isotopes most quickly?
(A) electron emission
(B) alpha particle emission
(C) gamma emission
(D) positron emission
The Attempt at a Solution
I assume they want you to convert the unstable isotopes to the non-radioactive isotopes (because they tell you the non-radioactive isotopes), so the goal is to add neutrons. I fail to see any emission which adds neutrons only. A results in higher atomic mass, B results in lower atomic mass, C does nothing, and D results in lower atomic mass.