Francium (87Fr) has no stable isotopes, so a standard atomic weight cannot be given. Its most stable isotope is 223Fr with a half-life of 22 minutes, occurring in trace quantities in nature as an intermediate decay product of 235U.
Of elements whose most stable isotopes have been identified with certainty, francium is the most unstable, though astatine is the rarest in nature due to its naturally occurring isotopes having even shorter half-lives and even smaller branching ratios.
In the case of all elements with atomic number 106 (seaborgium) and above, no isotopes have been discovered whose half-life exceeds that of francium-223. However, the isotopes of these elements so far discovered are all on the proton-rich side of beta stability; so there is a strong possibility that some undiscovered isotopes of these elements will have longer half-lives, which could exceed that of francium-223.
↑()– Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
↑#– Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
123#– Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
↑Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3) 030003. doi:10.1088/1674-1137/abddaf.
