Isotopes are any of the different forms of an element each having different atomic mass (mass number). Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons. Therefore, isotopes have different mass numbers, which give the total number of nucleons-the number of protons plus neutrons Isotopes such as radioisotopes, which are 'compounds containing radioactive forms of atoms, can be introduced into the body for the purpose of imaging, evaluating organ function, or localizing disease or tumors. Modern uses of isotopes that are useful to us are smoke detectors, irradiation in pest control, agriculture applications (radioactive tracers), medical uses, etc.