where mass number A equals to the sum of atomic number Z and number of neutrons N, and m p, m n, a V, a S, a C, a A are constants, one can see that the mass depends on Z and N non-linearly, even for a constant mass number. For odd A, it is admitted that δ = 0 and the mass dependence on Z is convex (or on N or N − Z, it does not matter for a constant A). This explains that beta decay is energetically favorable for neutron-rich nuclides, and positron decay is favorable for strongly neutron-deficient nuclides. Both decay modes do not change the mass number, hence an original nucleus and its daughter nucleus are isobars. In both aforementioned cases, a heavier nucleus decays to its lighter isobar.