Iodine is a chemical element with the symbol I and atomic number 53. It is classified as a halogen, which is a subset of very chemically reactive elements (Group 17 on the periodic table) that exist in the environment as compounds rather than as pure elements. The other halogens include fluorine (F), chlorine (Cl), bromine (Br), and astatine (At) . Iodine is the least reactive of the halogens as well as the most electropositive, meaning it tends to lose electrons and form positive ions during chemical reactions. It is also the heaviest and the least abundant of the stable halogens. Iodine is found in the Earths crust at a concentration of 0.14 ppm, making it the 64th most abundant element; it is also found in seawater at a concentration of 0.06 ppm. Iodine is an essential element needed for life, and it is best known for the vital role it plays in thyroid hormone production in humans as well as in all vertebrates. Iodine deficiency can lead to serious health problems, including goiter (enlarged thyroid gland), intellectual disability, and cretinism. Iodine is in Group 17 of the periodic table.