What Is Iodine?

Iodine is a halogen element with atomic number 53 and chemical symbol I, belonging to Group 17 of the periodic table. Under standard conditions it forms dark grey-to-black orthorhombic crystals with a characteristic metallic luster; when heated, these crystals sublime directly to a violet vapor without passing through a liquid phase at atmospheric pressure. Iodine is the heaviest stable halogen in common use and is less reactive than chlorine, bromine, or fluorine, a property that makes it useful in controlled-release and long-lived chemical applications. Its thermodynamic properties are documented in the NIST Chemistry WebBook, which provides vapor pressure data, thermochemical constants, and spectroscopic properties for the element.

Iodine occurs naturally in seawater, brine deposits, and caliche ore, with major commercial production centered in Chile and Japan. The element was isolated in 1811 by Bernard Courtois from seaweed ash. Its chemistry is governed by the ability to adopt oxidation states from -1 to +7, enabling a wide range of inorganic and organic iodine compounds with distinct reactivity and physical characteristics.

Physical and Chemical Properties

Elemental iodine has a molecular weight of 253.81 g/mol and exists as a diatomic molecule (I2) in solid and vapor phases. Its relatively low electronegativity among the halogens, combined with a large, polarizable electron cloud, makes it an effective oxidizing agent in mild conditions and a participant in charge-transfer complex formation with aromatic molecules such as starch, producing the characteristic blue-black color used as an indicator in analytical chemistry. The relatively low bond dissociation energy of I2 (151 kJ/mol) makes iodine accessible as a radical source under moderate thermal or photochemical activation, which is exploited in iodine-clock reactions and in photocatalytic synthesis.

Iodine in Electronics and Optical Materials

Iodine plays a specialized but important role in electronics and optical device manufacturing. In liquid crystal displays (LCDs), iodine is used to dope polyvinyl alcohol films that serve as polarizing filters. The iodine-doped polymer chains align to absorb light along one axis, creating the polarization necessary for LCD contrast. Iodine also modifies the refractive index of organic semiconductor crystals through halogen insertion into the crystal lattice; research in Scientific Reports on iodine insertion and refractive index in organic semiconductors demonstrates that halogen doping systematically tunes the optical response of organic single-crystal materials. Sodium iodide (NaI) doped with thallium, and cesium iodide (CsI), are widely used scintillator materials in radiation detectors, converting gamma-ray and X-ray photons into visible light pulses that photodetectors can measure.

Biological and Medical Applications

Iodine is an essential trace nutrient in human biology, required for the synthesis of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which regulate basal metabolic rate and developmental processes. Iodine deficiency is a leading preventable cause of intellectual disability worldwide; its elimination through iodized salt programs has been one of the most successful public health interventions of the twentieth century. The radioactive isotope iodine-131, with a half-life of approximately 8 days, is used both diagnostically and therapeutically in nuclear medicine: the thyroid gland preferentially concentrates iodine, allowing targeted irradiation of thyroid tissue. The NIH Office of Dietary Supplements collaborates with NIST on standard reference materials for iodine status assessment to support global analytical quality assurance programs.

Applications

Iodine has applications in a wide range of disciplines, including:

• LCD display manufacturing, where iodine-doped polymer films serve as polarizing filters
• Nuclear medicine and diagnostic imaging using radioactive iodine-131 and iodine-123
• Radiation detection and scintillator materials, including NaI(Tl) and CsI crystals
• Pharmaceutical synthesis as a catalyst and oxidant in organic chemistry
• Water treatment and antiseptic formulations for wound care and purification