Isobars are atoms of different chemical elements that have the same number of nucleons. Correspondingly, isobars differ in atomic number but have the same mass number. An example of a series of isobars would be 40S, 40Cl, 40Ar, 40K, and 40Ca. While the nuclei of these nuclides all contain 40 nucleons, they contain varying numbers of protons and neutrons. The term "isobars" for nuclides was suggested by Alfred Walter Stewart in 1918. It is derived from the Greek word isos, meaning "equal" and baros, meaning "weight".
Mass
The same mass number implies neither the same mass of nuclei, nor equal atomic masses of corresponding nuclides. From the Weizsäcker's formula for the mass of a nucleus where mass number equals to the sum of atomic number and number of neutrons , and,,,,, are constants, one can see that the mass depends on and non-linearly, even for a constant mass number. For odd , it is admitted that and the mass dependence on is convex. 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. For even the term has the form: where is another constant. This term, subtracted from the mass expression [|above], is positive for even-even nuclei and negative for odd-odd nuclei. This means that even-even nuclei, which have not a strong neutron excess or neutron deficiency, have higher binding energy than their odd-odd isobar neighbors. It implies that even-even nuclei are lighter and more stable. The difference is especially strong for small . This effect is also predicted by other nuclear models and has important consequences.
Stability
The Mattauch isobar rule states that if two adjacent elements on the periodic table have isotopes of the same mass number, at least one of these isobars must be a radionuclide. In cases of three isobars of sequential elements where the first and last are stable, branched decay of the middle isobar may occur; e.g. radioactive iodine-126 has an almost equal probabilities for two decay modes, which lead to different daughter isotopes: tellurium-126 and xenon-126. No observationally stable isobars exist for mass numbers 5, 8, 147, 151, as well as for 209 and above. Two observationally stable isobars exist for 36, 40, 46, 50, 54, 58, 64, 70, 74, 80, 84, 86, 92, 94, 96, 98, 102, 104, 106, 108, 110, 112, 114, 120, 122, 123, 124, 126, 132, 134, 136, 138, 142, 154, 156, 158, 160, 162, 164, 168, 170, 176, 180, 184, 192, 196, 198 and 204. In theory, no two stable nuclides have the same mass number, and no stable nuclides exist for mass number 5, 8, 143–155, 160–162, and ≥ 165, since in theory, the beta-decay stable nuclides for these mass number can undergo alpha decay.
