Iodine

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Iodine-triphenylphosphine charge-transfer complex in dichloromethane

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Iodine monochloride1

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Iodine is a chemical element with the symbol I and atomic number 53. It is a halogen in the 17th group of the periodic table; halogens are nonmetals that are five elements found in group 17. Iodine is the heaviest stable member of its group, with only tennessine (a synthetic element) being heavier, although tennessine is radioactive and only has short-lived isotopes. Iodine exhibits a shiny, black-metallic luster and a purple vapor when heated, characteristics that distinguish it from other halogens which are either gaseous or solid at room temperature.

Properties and Compounds

Iodine is unique among the halogens for being a solid at room temperature. It sublimates easily upon heating to form a beautiful violet vapor, a property that is both visually striking and useful for demonstration purposes in chemistry. Iodine is minimally soluble in water; however, it is highly soluble in a solution of potassium iodide (KI), forming a solution known as Lugol's iodine. It also dissolves well in organic solvents such as ethanol and chloroform.

Chemically, iodine is less reactive than the halogens above it in the periodic table, such as chlorine and bromine, but it forms compounds with many elements. The most commercially significant of these compounds is potassium iodate (KIO3), used in the iodization of salt to prevent iodine deficiency disorders.

Biological Role and Iodine Deficiency

Iodine is essential for life. It is a key component of thyroid hormones, which are critical for regulating metabolism, growth, and development in humans and other vertebrates. The thyroid gland absorbs iodine from the bloodstream to make its hormones, mainly thyroxine (T4) and triiodothyronine (T3).

Iodine deficiency is a significant health problem in parts of the world where the soil and food supply contain low levels of iodine. This deficiency can lead to various health issues, including goiter (an enlargement of the thyroid gland), cretinism in children (a condition associated with severely stunted physical and mental growth), and various degrees of other thyroid disorders. To combat iodine deficiency, many countries have implemented salt iodization programs, adding a small amount of potassium iodate to table salt.

Sources and Production

The primary sources of iodine are mineral deposits and seawater, with the largest producers being Chile and Japan. In mineral deposits, iodine is often found in the form of caliche, a type of sedimentary rock. Seawater contains iodine in low concentrations, and it is extracted through a process that involves the use of sulfur dioxide to reduce iodate solutions to elemental iodine, which is then evaporated and purified.

Uses

Beyond its critical biological role and use in nutrition, iodine has several industrial and medical applications. It is used in the production of LCD screens, photographic film, and as a catalyst in the synthesis of certain pharmaceuticals and synthetic fibers. In medicine, iodine is used as a disinfectant in various forms, including tincture of iodine, Povidone-iodine (an iodophor), and as a contrast agent in radiography.

Safety

While iodine is essential for health, excessive intake can lead to adverse effects, including the very thyroid problems it is taken to prevent. Handling elemental iodine also requires care, as it can cause burns upon contact with skin and is toxic if ingested in large amounts.