Iodine

Iodine is a chemical element with the symbol I and atomic number 53. It is a member of the halogen group. At room temperature it forms a purple-black solid that readily sublimes to a violet vapor. In nature it occurs mainly as iodide in seawater and marine organisms, and the element is commonly obtained from brine deposits and seaweed.

Chemically, iodine forms a range of iodide and iodate compounds and exists as diatomic molecules I2. It is less reactive than the lighter halogens but participates in numerous organic and inorganic synthesis reactions and can sublimate easily, giving a characteristic purple vapor.

Iodine is an essential trace element required for the synthesis of thyroid hormones thyroxine (T4) and triiodothyronine (T3). The thyroid concentrates iodine from the bloodstream, and deficiency can lead to goiter and impaired development, especially during pregnancy and early childhood. Typical adult dietary intake is around 150 micrograms per day; common sources include iodized salt, seafood, dairy products, and eggs.

Applications include antiseptic solutions such as tincture of iodine and iodophors, as well as fortification of salt with iodine to prevent deficiency. Radioactive isotopes of iodine (notably I-131, I-125, and I-123) are used in medical imaging and therapy for thyroid conditions.

History and safety: iodine was discovered in 1811 by Bernard Courtois and named from Greek ioeidēs meaning violet-colored. Excessive iodine intake can disrupt thyroid function and cause irritation; handling of radioactive isotopes requires appropriate medical supervision and safety protocols.