What are 3 general rules to predict the stability of an isotope

1) Calculate the total number of nucleons (protons and neutrons) in the nuclide
--> If the number of nucleons is even, there is a good chance it is stable.

2) Are there a magic number of protons or neutrons?
--> 2,8,20,28,50,82,114 (protons), 126 (neutrons), 184 (neutrons) are particularly stable in nuclei.

3) Calculate the N/Z ratio.
--> Use the belt of stability (Figure 1) to determine the best way to get from an unstable nucleus to a stable nucleus

Adozen is 1221/12 = 1.751.75 x 12 = 21therefore, 1.75 is the answer.