Beta decay does not change the number (A) of nucleons in the nucleus, but changes only its charge Z. Thus the set of all nuclides with the same A can be introduced; these isobaric nuclides may turn into each other via beta decay. For a given A 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 (A, Z) numbers, the neighbour nuclei (A, Z −1) and (A, Z +1) have higher mass excess and can beta decay into (A, Z), but not vice versa. For all odd mass numbers A, there is only one known beta-stable isobar. For even A, there are up to three different beta-stable isobars experimentally known; for example, 50 Sn, 52 Te, and 54 Xe are all beta-stable. There are about 350 known beta-decay stable nuclides.