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^Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three element gap of instability after polonium (84) where no isotopes have half-lives of at least four years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at a notable 1600 years, thus merits the element's inclusion here.
^This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
^Excluding those 'classically stable' isotopes with half-lives significantly in excess of 232Th, e.g. while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion.
