To determine the most stable isotope of element J based on its average atomic mass, you would need additional information such as the atomic number or the specific isotopic abundances. Without the atomic number or isotopic abundances, it is not possible to definitively identify the most stable isotope.
The average atomic mass of 212.01 amu suggests that there might be multiple isotopes present in varying abundances. Isotopes are atoms of the same element with different numbers of neutrons. The average atomic mass takes into account the masses of different isotopes and their relative abundances.
If you provide the atomic number of element J or the isotopic abundances, I can help you determine the most stable isotope based on that information.