Question

Predict whether each of the following nuclides is stable or unstable (radioactive). If the nuclide is unstable, predict the type of radioactivity you would expect it to exhibit.

I have the answers but don't understand part of them.

(a)    20^11_Na

Stable (I understand why, the mass is too light to be stable.)

But why are the types of radioactivity electron capture and positron production? Why not alpha decay too? Those three are all used when the nucleus is too light, so why is it just electron capture and positron production?

194^81_Tl has all three. What's the defining factor?

Thanks!

Answer 1

The nucleus is unstable if the neutron-proton ratio is less than 1:1 or greater than 1.5.

11Na²⁰  # neutrons = 9, # protons = 11

N/P ratio = 9/11 = 0.82 , So it is not stable.

The most stable isotope is 11Na²³, Where N/P ratio is 12/11= 1.09.

The nucleides comes inside the belt is stable.

3)

For Tl

N/P ratio is = 113/81 = 1.39

For heavier nucleides value should be 1.5. This can be achieved either by electron capture or by positron emission.