Radioactive Decay to a Non-radioactive Isotope

[Teemo]

Homework Statement

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

Homework Equations

N/A

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.

 

[Bystander]

I assume they want you to convert the unstable isotopes to the non-radioactive isotopes (because they tell you the non-radioactive isotopes)

Red herring. They're just telling you what isotopes of Mg are stable. You've also been told in class that mass number of a stable nucleus is usually at least twice the atomic number, in the case of Mg, 2 x 12 is 24, so the light isotopes are going to fall apart.

I fail to see any emission which adds neutrons only.

So, you've got to get rid of excess protons (positive charge) in the nucleus to get to a lower atomic number. Help?

 

[Teemo]

Sorry for the extremely late reply. But thanks, that makes so much more sense!