Biochemical pathways can be deciphered by using isotopic tracers. Isotopes are also the source of radiation used in medical diagnostic and treatment procedures. Note that each of these isotopes of hydrogen has only one proton. Isotopes differ from each other in the number of neutrons, not in the number of protons. Image from W.H. Freeman and Sinauer Associates, used by permission.
Some isotopes are radioisotopes, which spontaneously decay, releasing radioactivity .
Other isotopes are stable. Image from W.H. Freeman and Sinauer Associates, used by permission. The Periodic Table of the Elements provides a great deal of information about various elements. An on-line Periodic Table is available by clicking here, or if that is busy try a mirror site. The Periodic Table of the Elements.