Question about monoisotopic elements.... Of the 80 elements with at least one stable isotope, 26 have only one single stable isotope. The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element. The largest number of stable isotopes that occur for a single element is 10 (for tin, element 50). What does it mean for an element to have only one single stable isotope? I don't quite get it. I read that beryllium has only one stable isotope, which is beryllium-9. Does that mean that other than the normal beryllium, which has 4 protons and 4 neutrons and a mass number of 8, there is only one stable nuclide that isn't radioactive for beryllium? And what if beryllium-9 is called a stable isotope, what would the normal beryllium-8 with 4 protons and 4 neutons be called?