The masses of a set of isobars is given by:
Let us cast this in the form of the deviation from the mass of the most stable of this set. Putting and using ,
where is the mass of the most stable isobar (for the given A).
Thus for the odd-A nuclei (i.e. even-odd),
and for even-A nuclei
Thus the masses of odd-A nuclei are distributed along a parabola, while those for even-A nuclei alternate between two parabolas separated by . Since adjacent isobars are linked by beta decays, this has important consequences for the sequence of decays to the most stable isobar. In particular, there are likely to be two stable isobars for even A, having adjacent even Z and both being unable to decay to the odd Z isobar between them. For odd-A isobars there is only one stable isobar. (So, how many stable nuclei are there for ?).