See full list on physicsabout.com The Oxygen-18 isotope has an extra two neutrons, for a total of 10 neutrons and 8 protons, compared to the 8 neutrons and 8 protons in a normal oxygen atom. The slighty greater mass of 18 O—12.5 percent more than 16 O—results in differentiation of the isotopes in the Earth’s atmosphere and hydrosphere. By this definition, there are 256 known stable isotopes of the 80 elements which have one or more stable isotopes. A list of these is given at the end of this article. About two thirds of the elements have more than one stable isotope. One element (tin) has ten stable isotopes.