Complex sentence: Of the 80 elements with at least one stable isotope, 26 have only one stable isotope, and the mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element.
Split into simpler sentences: Of the 80 elements with at least one stable isotope, 26 have only one single stable isotop. The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element.