[{"from": "human", "value": "Could you give me a brief overview of this molecule? [129IH1]"}, {"from": "gpt", "value": "The molecule is a chemical element that has the symbol I and atomic number 53. Chemically, iodine is the second least reactive of the halogens, and the second most electropositive halogen; trailing behind astatine in both of these categories. However, the element does not occur in the free state in nature. As with all other halogens, when freed from its compounds iodine forms diatomic molecules. Iodine naturally occurs in the environment chiefly as a dissolved iodide in seawater, although it is also found in some minerals and soils. Iodine is an essential trace element for life, mainly as constituents of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). Iodine-129 is used for a variety of dating applications, including groundwater age dating and meteorite age dating. (L1845, L1847)"}]