the proton–neutron ratio is not the only factor affecting nuclear stability. it depends also on evenness or oddness of its atomic number "z", neutron number "n" and, consequently, of their sum, the mass number "a". oddness of both "z" and "n" tends to lower the [[nuclear binding energy]], making odd nuclei, generally, less stable. this remarkable difference of nuclear binding energy between neighbouring nuclei, especially of odd-"a" [[isobar (nuclide)|isobars]], has important consequences: unstable isotopes with a nonoptimal number of neutrons or protons decay by [[beta decay]] (including positron decay), [[electron capture]] or other exotic means, such as [[spontaneous fission]] and [[cluster decay]].