Iodine (chemical element)

Physical and chemical characteristics

Iodine crystals are brittle and dark gray to black; when heated they sublimate, producing a characteristic violet vapor and purple vapour whose color contributed to the element's name (from the Greek for violet). At the atomic level its outer electrons give it a valence electron configuration ending in 5p5, which makes it an oxidizing element that readily accepts electrons to form iodide (I−). In aqueous chemistry iodine exists in several forms including iodide, iodate and organic iodine-containing compounds. The naturally abundant, stable isotope is 127I; several radioactive isotopes (notably 131I) are produced artificially and used in medicine and research.

Occurrence and history

Iodine is relatively rare in Earth's crust but concentrated in seawater, marine plants and certain mineral deposits. Historically it was isolated from seaweed ash and later obtained from brines and nitrate ores. The element was first identified in 1811 by Bernard Courtois and named for the violet color of its vapor; the naming and early characterization involved chemists who recognized its distinct properties. Natural cycles and deposition in soils influence regional iodine availability in food and drinking water.

Uses and importance

• Nutritional: Iodine is essential for human and animal health because it is required to make thyroid hormones (triiodothyronine, T3, and thyroxine, T4). Many countries add iodine to table salt (iodized salt) to prevent deficiency and related goiter and developmental disorders.
• Medical: Radioactive iodine isotopes are used to diagnose and treat thyroid disorders; nonradioactive iodine solutions and povidone-iodine are widely used as topical antiseptics.
• Industrial and laboratory: Iodine compounds serve as catalysts, reagents in organic synthesis, and components in photographic chemicals and certain disinfectants. Some specialized industrial processes recover iodine from brine and mineral sources.

Safety, environmental and notable facts

In small amounts iodine is essential, but in excess it can be toxic and irritant; handling precautions apply to concentrated solutions, iodine vapor, and radioactive isotopes. Iodine deficiency remains a public health issue in some inland and mountainous regions, remedied through fortification programs. Because of its volatility and biological activity, iodine is also a focus of environmental monitoring when released from medical or industrial sources.

For further general reference on its elemental classification see chemical element entries and the periodic table. Additional resources on nutrition, medical uses and safety guidance can be consulted via dedicated public health and chemical safety sources (basic nuclear and isotope information, elemental profiles, group chemistry, and isotopic data).