which is the correct way to write iodine (i) with an atomic mass of 131
Iodine-131 Iodine-131 (131I) is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley.[1] It has a radioactive decay half-life of about eight days. It is associated with nuclear energy, medical diagnostic and treatment procedures, and natural gas production. It also plays a major role as a radioactive isotope present in nuclear fission products, and was a significant contributor to the health hazards from open-air atomic bomb testing in the 1950s, and from the Chernobyl disaster, as well as being a large fraction of the contamination hazard in the first weeks in the Fukushima nuclear crisis. This is because I-131 is a major fission product of uranium and plutonium, comprising nearly 3% of the total products of fission (by weight). See fission product yield for a comparison with other radioactive fission products. I-131 is also a major fission product of uranium-233, produced from thorium.
Iodine test The iodine test is used to test for the presence of starch. Starch turns an intense "blue-black" colour upon addition of aqueous solutions of the triiodide anion, due to the formation of an intermolecular charge-transfer complex. In the absence of starch, the brown colour of the aqueous solution remains. This interaction between starch and triiodide is also the basis for iodometry.