Iodine
Iodine is a chemical element with symbol I and atomic number 53. The name is from Greek ἰοειδής ioeidēs, meaning violet or purple, due to the color of iodine vapor. Iodine and its compounds are primarily used in nutrition, and industrially in the production of acetic acid and certain polymers.
Identity
- Atomic mass
- 126.904 u
- Category
- Halogens
- Group
- 17
- Period
- 5
- Block
- p
- Oxidation States
- −1, +1, +3, +5, +7
- Phase (25 °C)
- Solid
- Appearance
- lustrous metallic gray, violet as a gas
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5
- Short configuration
- [Kr] 4d10 5s2 5p5
- Electrons per shell
- 2, 8, 18, 18, 7
- Electronegativity (Pauling)
- 2.66
- Electron affinity
- 295 kJ/mol
- 1st ionization energy
- 1.01e+3 kJ/mol
- Ionization energies
- 1008 · 1846 · 3180 kJ/mol
Physical properties
- Melting point
- 113.7 °C (387 K)
- Boiling point
- 184.3 °C (457 K)
- Density
- 4.933 g/cm³
- Molar heat capacity
- —
Chemical behavior of iodine
Iodine has seven valence electrons in its outer shell ([Kr] 4d¹⁰ 5s² 5p⁵). Its most frequent and stable oxidation state is -1 in iodides, but because its electronegativity is lower and its polarizability greater than its lighter halogens, it shows a rich chemistry in positive oxidation states (+1, +3, +5 and +7), forming oxoanions such as iodate (IO₃⁻) and periodate (IO₄⁻). It is a solid non-metal with a metallic grey-violet shine that sublimes easily into purple vapour. It is the least reactive stable halogen and the weakest oxidising agent of its group, but it acts as an excellent nucleophile in polar solvents.
Reactivity
Cl₂ + 2 KI → 2 KCl + I₂
It is easily displaced from its salts by more electronegative halogens such as chlorine or bromine, releasing elementary solid iodine or in solution.
I₂ + I⁻ ⇌ I₃⁻
It is poorly soluble in pure water, but dissolves abundantly in potassium iodide aqueous solutions forming the dark brown triiodide complex (Lugol).
2 Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2 NaI
It reacts quantitatively with sodium thiosulfate in the classic iodometric titration, changing colour from brown to transparent.
I₂ + starch → coloured complex
It inserts into the helical structure of starch forming an unmistakable dark blue charge-transfer complex.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Potassium iodide | KI | Iodised salt supplement to prevent goitre, thyroid protector against nuclear emergencies and chemical reagent |
| Povidone-iodine | C₆H₉I₂NO | Broad-spectrum antiseptic and disinfectant to heal wounds and prepare the surgical skin |
| Potassium iodate | KIO₃ | Iodisation agent of table salt in tropical climates by its superior thermal and oxidative stability |
| Diiodomethane | CH₂I₂ | Highly dense organic liquid used in gemology to determine mineral densities by flotation |
Electron shell distribution
View in the atom builderIsotopes
Natural iodine is strictly monoisotopic, composed of 100% of stable iodine-127 (¹²⁷I, with 74 neutrons). More than thirty radioisotopes have been synthesised; among them, iodine-131 (¹³¹I, half-life of 8.02 days) is the pillar of radioisotope therapy against thyroid cancer and hyperthyroidism. Iodine-129 (¹²⁹I, half-life of 15.7 million years) is a long-lived nuclear fission product used as an environmental and geological tracer.
Occurrence and production
It is a relatively rare element in the lithosphere (about 0.46 ppm), but it is enriched in saline deposits and in the marine biosphere. It does not appear free. Its primary global sources are the evaporite deposits of caliche in the Atacama Desert in Chile (where it appears as the mineral lautarite, Ca(IO₃)₂, and dietzeite), and underground brines associated with natural gas fields in Japan and the United States, where iodine is found as dissolved iodide ion. It is extracted by oxidising brines with chlorine or sulphur dioxide and blowing iodine vapor with air.
History and origin of the name
It was discovered in 1811 by the French chemist Bernard Courtois while producing saltpetre for Napoleon's war gunpowder from seaweed ash. On adding an excess of sulphuric acid to the leachates to clean copper vessel residues, he observed the appearance of dense intense violet vapours that condensed into brilliant dark crystals. Joseph Louis Gay-Lussac characterised it as a new element in 1813 and gave it the name 'iodine', from the Greek 'ioeidês' (violet-coloured), due to the characteristic colour of its vapour.
Applications and relevance of iodine
Uses of the element
Elemental iodine is the indispensable reagent for iodometric analytical titrations and for the synthesis of acetic acid by the industrial Monsanto process and Cativa processes (with rhodium or iridium catalysts promoted by methyl iodide). It is used in quartz halogen lamps to regenerate the tungsten filament, in optical polarisers of LCD screens and in water disinfection on expeditions and spacecraft.
Uses of its compounds
In medicine, iodinated organic compounds are the most widely used radiological contrast media in computed tomography (CT) and X-ray angiographies due to their high atomic number. Povidone-iodine is the reference skin antiseptic; potassium iodide universalises iodised table salt; and silver iodide is used in cloud seeding.
Health, safety and environment
Biological role
It is a strictly essential trace element for humans and vertebrates. It forms the molecular nucleus of thyroid hormones thyroxine (T₄) and triiodothyronine (T₃), responsible for regulating basal metabolism, physical growth and fetal and childhood brain development. Its deficiency causes endemic goitre and cretinism.
Safety
Solid elemental iodine and its vapours are irritants and corrosives to the eyes, skin and respiratory mucous membranes, and can cause chemical burns on prolonged contact and tracheal inflammation if concentrated vapours are inhaled. In case of a nuclear accident, preventive ingestion of potassium iodide tablets saturates the thyroid gland, preventing it from absorbing the volatile radioactive iodine-131.
Environment
Iodine circulates naturally through the biogeochemical ocean-atmosphere-soil cycle by the emission of iodomethane by microscopic marine algae. It does not present ecotoxic problems except for the accidental discharge of radioisotopes from nuclear installations.
Frequently asked questions about Iodine
Why is potassium iodide recommended in some nuclear emergencies?
The thyroid needs iodine to produce hormones and does not distinguish between stable and radioactive iodine. If authorities recommend it, taking potassium iodide provides stable iodine and reduces the absorption of radioactive iodine by the thyroid. Thus the risk of thyroid cancer associated with exposure can be reduced. However, it does not protect against other radioactive materials or against all radiation, and should only be taken following health authority instructions.
Why is iodine used to improve the images of some medical tests?
Iodine absorbs X-rays more efficiently than many soft tissues of the body. That is why contrast media containing iodine allow better differentiation of blood vessels and certain organs in a computed tomography (CT). As they circulate through the body, they increase image contrast and help medical staff detect alterations that could go unnoticed. Its administration requires assessing possible contraindications and the health status of each patient.
Why do halogen bulbs last longer than traditional incandescent bulbs?
In an incandescent bulb, part of the tungsten of the filament evaporates with heat and can deposit on the glass, darkening it. In halogen bulbs, a small amount of iodine or another halogen participates in a chemical cycle that allows returning part of that tungsten to the filament. This process delays deterioration and allows the bulb to operate at high temperatures. The result is intense light and generally a longer overall lifespan than conventional incandescent bulbs.
Why do some marine algae contain so much iodine?
Some brown algae can concentrate large amounts of iodine by absorbing it from seawater and storing it in their tissues. This element participates in chemical processes that help algae respond to certain environmental conditions, although their exact functions vary according to the species. This capacity explains why some algae, such as kelp, can contain much more iodine than other foods. It is also important to control its consumption, as an excess of iodine can alter thyroid function.
Why is almost all cooking salt enriched with iodine?
The thyroid needs small amounts of iodine to produce the hormones that control our metabolism, growth and brain development. As foods grown in iodine-poor soils lack this element, adding a pinch of iodide to common salt has been one of the most successful public health measures in history to prevent goitre and childhood neurological problems.
Why does iodine change colour when it touches foods such as potato or bread?
The starch present in potatoes, rice or flour is formed by long chains of carbohydrates folded in a spiral or helix. When a few drops of iodine solution are added, the molecules of this element insert perfectly into the inner space of the helix. This physical union alters the way light interacts with the mixture, producing a dark almost black blue colour.
Why does the thyroid need iodine to function correctly?
The thyroid needs iodine to make two fundamental hormones: thyroxine (T4) and triiodothyronine (T3). These hormones help regulate metabolism, growth and the development of the body. The gland captures iodine from the blood and incorporates it into a protein that serves as a base for producing both hormones. Insufficient intake can make it difficult to produce them, while consuming too much iodine can also cause thyroid disorders.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.