Four elements have seven stable isotopes , eight have six stable isotopes , ten have five stable isotopes , nine have four stable isotopes , five have three stable isotopes , 16 have two stable isotopes ( counting 180m 73Ta as stable ) , and 26 elements have only a single stable isotope ( of these , 19 are so-called mononuclidic elements , having a single primordial stable isotope that dominates and fixes the atomic weight of the natural element to high precision ; 3 radioactive mononuclidic elements occur as well ) .
Hydrogen has three common isotopes .