iodine

iodine
Name	iodine
Number	53
Category	halogen
Appearance	lustrous metallic gray solid, violet vapor
Standard atomic weight	126.90447
Electron configuration	[Kr] 4d10 5s2 5p5
Phase	solid
Melting point c	113.7
Boiling point c	184.3
Discovered by	Bernard Courtois
Discovery date	1811

iodine is a chemical element with the symbol I and atomic number 53. It is the heaviest of the stable halogens, appearing as a lustrous, metallic-gray solid at room temperature that sublimes into a striking violet-pink gas. The element was first isolated in 1811 by Bernard Courtois from the ashes of seaweed. Its most critical role is in biology, where it is an essential trace element for the synthesis of thyroid hormones by the thyroid gland.

Properties

iodine is a nonmetal and a member of the halogen group on the periodic table, sharing properties with fluorine, chlorine, and bromine. It has a distinct violet color as a gas, which is rare among the elements, and forms diatomic molecules (I₂). It is less reactive than the lighter halogens but still forms compounds with many elements, such as hydrogen iodide and various iodates. Iodine dissolves in solvents like ethanol to give a brown solution and in carbon tetrachloride or chloroform to give a violet solution, a property used in the iodine test for starch. It exhibits a range of oxidation states, with the most common being -1, +5, and +7, seen in compounds like potassium iodide and periodic acid.

History

The element was discovered in 1811 during the Napoleonic Wars by French chemist Bernard Courtois, who was extracting saltpeter for gunpowder from seaweed ash. He noticed a violet vapor that condensed into dark crystals, which were subsequently investigated by chemists like Joseph Louis Gay-Lussac and Humphry Davy. Gay-Lussac identified it as a new element and named it "iode" from the Greek *iodes*, meaning violet. Its antiseptic properties were pioneered by Ignaz Semmelweis in obstetrics and later by Joseph Lister in surgery. The importance of iodine in preventing goiter was established in the early 20th century, leading to public health initiatives like iodized salt.

Occurrence and production

iodine is relatively rare in the Earth's crust but is found in certain minerals and brines. It occurs primarily as the iodide ion in seawater, albeit at low concentrations, and is concentrated by certain sea life like kelp and seaweed. Commercially, it is mainly produced from caliche deposits in the Atacama Desert of Chile and from brine wells in Japan and the United States, particularly in Oklahoma. The production process often involves oxidizing iodides to iodine, frequently using chlorine. Major global producers include companies like SQM in Chile and Iofina in the U.S.

Applications

The single largest use of iodine is in the production of X-ray contrast media for medical imaging procedures. It is a key component in tincture of iodine and other antiseptics, though its use has declined in favor of other agents like povidone-iodine. Iodine compounds are used as catalysts in the production of acetic acid and other chemicals, and silver iodide is employed in cloud seeding. It serves as a nutritional supplement in animal feed and is critical in the manufacture of LCD screens for televisions and phones. Historically, it was used in photographic film as silver iodide.

Biological role and health effects

iodine is an essential micronutrient for all animals and some plants, being a fundamental constituent of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones, produced by the thyroid gland, regulate metabolism, growth, and development. Iodine deficiency is the world's leading preventable cause of intellectual disability and can lead to goiter, hypothyroidism, and conditions like cretinism. Adequate intake is typically achieved through dietary sources like seafood, dairy products, and iodized salt. The recommended daily intake is monitored by organizations like the World Health Organization.

Precautions

Elemental iodine is toxic and can be corrosive in both solid and vapor forms; inhalation of its vapor irritates the eyes, lungs, and mucous membranes. Direct contact with skin can cause stains and lesions, and excessive intake can lead to iodism, with symptoms including mouth burning, fever, and gastric upset. Radioactive isotopes, such as iodine-131 produced during nuclear fission, pose a significant health risk if absorbed by the thyroid, a concern highlighted by events like the Chernobyl disaster and the Fukushima Daiichi nuclear disaster. In such cases, stable potassium iodide tablets are administered as a blocking agent.

Category:Chemical elements Category:Halogens Category:Dietary minerals