Iodine
Iodine (pronounced /ˈaɪ.ɵdaɪn/ EYE-o-dyne, /ˈaɪ.ɵdɨn/ EYE-o-dən, or in chemistry /ˈaɪ.ɵdiːn/ EYE-o-deen; from Greek: ιώδης iodes meaning violet (or purple), is a chemical element that has the symbol I and the atomic number 53.
Chemically, iodine is the second least reactive of the halogens, and the second most electropositive halogen, trailing astatine in both aspects. However, the element does not occur in the free state in nature. As with all other halogens (members of Group 17 in the periodic table), when freed from its compounds iodine forms diatomic molecules (I2).
Iodine and its compounds are primarily used in medicine, photography, and dyes. Iodine is rare in the solar system and Earth’s crust; however, the iodides are very soluble in water and the element concentrates in seawater, where it occurs in greater concentrations than in rocks. This mechanism helps to explain how the element came to be required in trace amounts by all animals and some plants, being the heaviest element commonly used by living organisms (only tungsten, used in enzymes by a few bacteria, is heavier).