This number does not change. The number of neutrons may vary. Whenever this happens, an element is said to exist in several forms known as isotopes.
For example, the hydrogen atom has one electron, one proton and no neutrons. deuterium, one of its isotopes, has one electron, one proton, and one neutron. Tritium, its other isotope, has one neutron more. uranium has three isotopes but only one, Uranium 235, undergoes fission to create energy.
A radioisotope is an isotope which gives off radiation. The most effective radioisotopes are the ones which are made from atomic piles. The rays which radioisotopes emit can be detected and traced by a geiger counter, and therefore they serve a variety of useful purposes in medicine, in industry, and in research. Radioisotopes used for this purpose are called tracers.
