Chemistry. Chemical element with the symbol I. Iodine is a nonmetallic element of the halogen group. It is an essential trace element for all organisms. It occurs in very small amounts in nature, especially in seaweed and see water. Iodine is used by the thyroid gland to build up the thyroid hormones thyroxine and triiodothyronine. Iodine compounds are used as antiseptics and dyes.
The radioactive isotope iodine-131 is used as a medical tracer and in therapy.