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1. Jan 6, 2015

### Teemo

1. The problem statement, all variables and given/known data
Magnesium (Z = 12) has isotopes that range from Mg–20
to Mg–31. Only Mg–24, Mg–25, and Mg–26 are not
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

2. Relevant equations
N/A

3. 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.

2. Jan 6, 2015

### Bystander

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

3. Jan 18, 2015

### Teemo

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