The Mattauch isobar rule, formulated by Josef Mattauch in 1934, states that if two adjacent elements on the periodic table have isotopes of the same mass number, one of the isotopes must be radioactive.
[1] A consequence of this rule is that technetium and promethium both have no stable isotopes, as each of the neighboring elements on the periodic table (molybdenum and ruthenium, and neodymium and samarium, respectively) have a beta-stable isotope for each mass number for the range in which the isotopes of the unstable elements usually would be stable to beta decay.
[1][2] These ranges can be calculated using the liquid drop model (for example the stability of technetium isotopes), in which the isobar with the lowest mass excess or greatest binding energy is shown to be stable to beta decay[3] because energy conservation forbids a spontaneous transition to a less stable state.
The only known exceptions to the Mattauch isobar rule are the cases of antimony-123 and tellurium-123 and of hafnium-180 and tantalum-180m, where both nuclei are observationally stable.
As mentioned above, the Mattauch isobar rule cannot make predictions as to the half-lives of the beta-unstable isotopes.