Beta-decay stable isobars are the set of nuclides which cannot undergo beta decay, that is, the transformation of a neutron to a proton or a proton to a neutron within the nucleus. A subset of these nuclides are also stable with regards to double beta decay or theoretically higher simultaneous beta decay, as they have the lowest energy of all isobars with the same mass number.

This set of nuclides is also known as the line of beta stability, a term already in common use in 1965.^[1][2] This line lies along the bottom of the nuclear valley of stability.

The line of beta stability can be defined mathematically by finding the nuclide with the greatest binding energy for a given mass number, by a model such as the classical semi-empirical mass formula developed by C. F. Weizsäcker. These nuclides are local maxima in terms of binding energy for a given mass number.

All odd mass numbers have only one beta decay stable nuclide.

Among even mass number, five (124, 130, 136, 150, 154) have three beta-stable nuclides. None have more than three; all others have either one or two.

• From 2 to 34, all have only one.
• From 36 to 72, only eight (36, 40, 46, 50, 54, 58, 64, 70) have two, and the remaining 11 have one.
• From 74 to 122, three (88, 90, 118) have one, and the remaining 22 have two.
• From 124 to 154, only one (140) has one, five have three, and the remaining 10 have two.
• From 156 to 262, only eighteen have one, and the remaining 36 have two, though there may also exist some undiscovered ones.

All primordial nuclides are beta decay stable, with the exception of ⁴⁰K, ⁵⁰V, ⁸⁷Rb, ¹¹³Cd, ¹¹⁵In, ¹³⁸La, ¹⁷⁶Lu, and ¹⁸⁷Re. In addition, ¹²³Te and ^180mTa have not been observed to decay, but are believed to undergo beta decay with an extremely long half-life (over 10¹⁵ years). (¹²³Te can only undergo electron capture to ¹²³Sb, whereas ^180mTa can decay in both directions, to ¹⁸⁰Hf or ¹⁸⁰W.) Among non-primordial nuclides, there are some other cases of theoretically possible but never-observed beta decay, notably including ²²²Rn and ²⁴⁷Cm (the most stable isotopes of their elements considering all decay modes). Finally, ⁴⁸Ca and ⁹⁶Zr have not been observed to undergo beta decay (which is theoretically possible for both), but double beta decay is known for both. All elements up to and including nobelium, except technetium and promethium, are known to have at least one beta-stable isotope.

350 beta-decay stable nuclides are currently known.^[3][4] Theoretically predicted or experimentally observed double beta-decay is shown by arrows, i.e. arrows point towards the lightest-mass isobar. This is sometimes dominated by alpha decay or spontaneous fission, especially for the heavy elements. Possible decay modes are listed as α for alpha decay, SF for spontaneous fission, and n for neutron emission in the special case of ⁵He. For mass 5 there are no bound isobars at all; there are bound isobars for mass 8, but the beta-stable one ⁸Be is unbound.^[5]

Two beta-decay stable nuclides exist for odd neutron numbers 1 (²H and ³He), 3 (⁵He and ⁶Li – the former having an extremely short half-life), 5 (⁹Be and ¹⁰B), 7 (¹³C and ¹⁴N), 55 (⁹⁷Mo and ⁹⁹Ru), and 85 (¹⁴⁵Nd and ¹⁴⁷Sm); the first four cases involve very light nuclides where odd-odd nuclides are more stable than their surrounding even-even isobars, and the last two surround the proton numbers 43 and 61 which have no beta-stable isotopes. Also, two beta-decay stable nuclides exist for odd proton numbers 1, 3, 5, 7, 17, 19, 29, 31, 35, 47, 51, 63, 77, 81, and 95; the first four cases involve very light nuclides where odd-odd nuclides are more stable than their surrounding even-even isobars, and the other numbers surround the neutron numbers 19, 21, 35, 39, 45, 61, 71, 89, 115, 123, 147 which have no beta-stable isotopes. (For N = 21 the long-lived primordial ⁴⁰K exists, and for N = 71 there is ¹²³Te whose electron capture has not yet been observed, but neither are beta-stable.)

All even proton numbers 2 ≤ Z ≤ 102 have at least two beta-decay stable nuclides, with exactly two for Z = 4 (⁸Be and ⁹Be – the former having an extremely short half-life) and 6 (¹²C and ¹³C). Also, the only even neutron numbers with only one beta-decay stable nuclide are 0 (¹H) and 2 (⁴He); at least two beta-decay stable nuclides exist for even neutron numbers in the range 4 ≤ N ≤ 160, with exactly two for N = 4 (⁷Li and ⁸Be), 6 (¹¹B and ¹²C), 8 (¹⁵N and ¹⁶O), 66 (¹¹⁴Cd and ¹¹⁶Sn, noting also primordial but not beta-stable ¹¹⁵In), 120 (¹⁹⁸Pt and ²⁰⁰Hg), and 128 (²¹²Po and ²¹⁴Rn – both very unstable to alpha decay). Seven beta-decay stable nuclides exist for the magic N = 82 (¹³⁶Xe, ¹³⁸Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴²Nd, and ¹⁴⁴Sm) and five for N = 20 (³⁶S, ³⁷Cl, ³⁸Ar, ³⁹K, and ⁴⁰Ca), 50 (⁸⁶Kr, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, and ⁹²Mo, noting also primordial but not beta-stable ⁸⁷Rb), 58 (¹⁰⁰Mo, ¹⁰²Ru, ¹⁰³Rh, ¹⁰⁴Pd, and ¹⁰⁶Cd), 74 (¹²⁴Sn, ¹²⁶Te, ¹²⁷I, ¹²⁸Xe, and ¹³⁰Ba), 78 (¹³⁰Te, ¹³²Xe, ¹³³Cs, ¹³⁴Ba, and ¹³⁶Ce), 88 (¹⁴⁸Nd, ¹⁵⁰Sm, ¹⁵¹Eu, ¹⁵²Gd, and ¹⁵⁴Dy – the last not primordial), and 90 (¹⁵⁰Nd, ¹⁵²Sm, ¹⁵³Eu, ¹⁵⁴Gd, and ¹⁵⁶Dy).

For A ≤ 209, the only beta-decay stable nuclides that are not primordial nuclides are ⁵He, ⁸Be, ¹⁴⁶Sm, ¹⁵⁰Gd, and ¹⁵⁴Dy. (¹⁴⁶Sm has a half-life long enough that it should barely survive as a primordial nuclide, but has never been experimentally confirmed as such.)

All beta-decay stable nuclides with A ≥ 209 are known to undergo alpha decay, though for some, spontaneous fission is the dominant decay mode. Cluster decay is sometimes also possible, but in all known cases it is a minor branch compared to alpha decay or spontaneous fission. Alpha decay is energetically possible for all beta-stable nuclides with A ≥ 165 with the single exception of ²⁰⁴Hg, but in most cases the Q-value is small enough that such decay has never been seen.^[11] With the exception of ²⁶²No, no nuclides with A > 260 have been definitively identified as beta-stable. ²⁶⁰Fm and ²⁶²No are unconfirmed.^[4] Moreover, the known beta-stable nuclei for individual masses A > 257 may not represent the complete set.^[10][12]

The general patterns of beta-stability are expected to continue into the region of superheavy elements, though the exact location of the center of the valley of stability is model dependent. It is widely believed that an island of stability exists along the beta stability line for isotopes of elements around copernicium that are stabilized by shell closures in the region; such isotopes would decay primarily through alpha decay or spontaneous fission.^[13] Beyond the island of stability, various models that correctly predict many known beta-stable isotopes also predict anomalies in the beta-stability line that are unobserved in any known nuclides, such as the existence of two beta-stable nuclides with the same odd mass number.^[10][14] This is a consequence of the fact that a semi-empirical mass formula must consider shell correction and nuclear deformation, which become far more pronounced for heavy nuclides.^[14][15]

Beta decay generally causes nuclides to decay toward the isobar with the lowest mass (which is often, but not always, the one with highest binding energy) with the same mass number. Those with lower atomic number and higher neutron number than the minimum-mass isobar undergo beta-minus decay, while those with higher atomic number and lower neutron number undergo beta-plus decay or electron capture.

However, there are a few odd-odd nuclides between two beta-stable even-even isobars, that predominantly decay to the higher-mass of the two beta-stable isobars. For example, ⁴⁰K could either undergo electron capture or positron emission to ⁴⁰Ar, or undergo beta minus decay to ⁴⁰Ca: both possible products are beta-stable. The former process would produce the lighter of the two beta-stable isobars, yet the latter is more common.

• Isotope masses from:
  • Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.