Iodine

Iodine (IPA: , /ˈaɪəˌdɪn/, or /ˈaɪəˌdiːn/; from Greek: iodes "violet"), is a chemical element that has the symbol I and atomic number 53. Chemically, iodine is the least reactive of the halogens, and the most electropositive halogen after astatine. Iodine is primarily used in medicine, photography and dyes. It is required in trace amounts by most living organisms.

As with all other halogens (members of Group VII in the Periodic Table), iodine forms diatomic molecules, and hence, has the molecular formula of I2.