text: 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. (Note that although Sm is unstable, it is stable to beta decay; thus 147 is not a counterexample). 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 because energy conservation forbids a spontaneous transition to a less stable state.
  Thus no stable nuclides have proton number 43 or 61, and by the same reasoning no stable nuclides have neutron number 19, 21, 35, 39, 45, 61, 71, 89, 115, or 123.