If the proton and neutron are part of an atomic nucleus, the above described decay processes transmute one chemical element into another. For example:
Beta decay does not change the number () of nucleons in the nucleus, but changes only its charge . Thus the set of all nuclides with the same  can be introduced; these isobaric nuclides may turn into each other via beta decay. For a given  there is one that is most stable. It is said to be beta stable, because it presents a local minimum of the mass excess: if such a nucleus has  numbers, the neighbour nuclei  and  have higher mass excess and can beta decay into , but not vice versa. For all odd mass numbers , there is only one known beta-stable isobar. For even , there are up to three different beta-stable isobars experimentally known; for example, , , and  are all beta-stable. There are about 350 known beta-decay stable nuclides.