Topics: How do you find the relative atomic? mass of an element?
There are 37 known isotopes of iodine ( 53 I ) from 108 I to 144 I; all undergo radioactive decay except 127 I, which is stable. Iodine is thus a monoisotopic element.
Its longest-lived radioactive isotope , 129 I, has a half-life of 15.7 million years, which is far too short for it to exist as a primordial nuclide. Cosmogenic sources of 129 I produce very tiny quantities of it that are too small to affect atomic weight measurements; iodine is thus also a mononuclidic element —one that is found in nature only as a single nuclide. Most 129 I derived radioactivity on Earth is man-made, an unwanted long-lived byproduct of early nuclear tests and nuclear fission accidents.
All other iodine radioisotopes have half-lives less than 60 days, and four of these are used as tracers and therapeutic agents in medicine. These are 123 I, 124 I, 125 I, and 131 I. All industrial production of radioactive iodine isotopes involves these four useful radionuclides.