[LHM]

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

I know that it's not gamma emission, and I also know that the proton to neutron ratio is too large, right? And alpha particle emission isn't going to decrease the ratio. But how do you know if it's electron emission or positron emission?

[vmelkon]

When you have stable isotopes like 24, 25, 26 in something natural, then it is highly likely that the other isotopes are radioactive.
24, 25, 26 are the central points of magnesium.
Anything lower will decay by position emission. 20, 21, 22, 23.
proton -> neutron + positron + neutrino

Anything higher will decay by electron emission. 27 and above.
neutron -> proton + electron + anti-neutrino

Of course, there are other modes of decay possible like K capture, double beta decay, and perhaps others I haven't heard of.
K capture happens if there isn't enough energy in the nucleus thus positron emission can't occur. It is a very curious phenomenon.