Article Outline Radioactive isotopes. Uses of radioisotopes. Some elements, such as aluminum, fluorine, gold, and phosphorus, have only one naturally occurring isotope. All atoms of these elements have the same weight.
Most elements have several naturally occurring isotopes.
For example, hydrogen has three isotopes. This particle is a proton, and so it has a positive charge. The second isotope, called deuterium or heavy hydrogen, has a mass number of 2. The neutron has almost the same mass as the proton. The heaviest hydrogen isotope, tritium, has a mass number of 3 and is radioactive.