Isotopes of francium

Francium (Fr) has no stable isotopes. A standard atomic mass cannot be given. Its most stable isotope is ²²³Fr with a half-life of 22 minutes, occurring in trace quantities as an intermediate decay product of ²³⁵U. The other natural isotope is ²²¹Fr.

Of elements whose most stable isotopes have been identified with certainty, francium is the most unstable. All elements with atomic number of greater than or equal to 106 (seaborgium) have most-stable-known isotopes shorter than that of francium, but those elements have only a relatively small number of isotopes discovered, thus, there is the possibility of a yet-unknown isotope having a longer half-life.