View source for Iodine

{{About|the chemical element}}
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{{Infobox iodine|engvar=en-GB}}

'''Iodine''' is a [[chemical element]]; it has [[Chemical symbol|symbol]] '''I''' and [[atomic number]] 53. The heaviest of the stable [[halogen]]s, it exists at [[Standard temperature and pressure|standard conditions]] as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at {{convert|114|C}}, and boils to a violet gas at {{convert|184|C}}. The element was discovered by the French chemist [[Bernard Courtois]] in 1811 and was named two years later by [[Joseph Louis Gay-Lussac]], after the [[Ancient Greek]] {{lang|grc|Ιώδης}}, meaning 'violet'.

Iodine occurs in many oxidation states, including [[iodide]] (I<sup>−</sup>), [[iodate]] ({{chem|IO|3|-}}), and the various [[periodate]] anions. As the heaviest essential [[Mineral (nutrient)|mineral nutrient]], iodine is required for the synthesis of [[thyroid hormones]].<ref name="lpi">{{cite web|url=http://lpi.oregonstate.edu/mic/minerals/iodine|title=Iodine|publisher=Micronutrient Information Center, [[Linus Pauling Institute]], [[Oregon State University]], Corvallis|date=2015|access-date=20 November 2017|archive-date=17 April 2015|archive-url=https://web.archive.org/web/20150417055246/http://lpi.oregonstate.edu/mic/minerals/iodine|url-status=live}}</ref> [[Iodine deficiency]] affects about two billion people and is the leading preventable cause of [[Intellectual disability|intellectual disabilities]].<ref>{{cite news|url= https://query.nytimes.com/gst/fullpage.html?res=9E05E3D81231F935A25751C1A9609C8B63|work=The New York Times|title=In Raising the World's I.Q., the Secret's in the Salt| vauthors = McNeil Jr DG |date=16 December 2006|access-date=21 July 2009|url-status=live|archive-url= https://web.archive.org/web/20100712011551/http://query.nytimes.com/gst/fullpage.html?res=9E05E3D81231F935A25751C1A9609C8B63|archive-date=12 July 2010}}</ref>

The dominant producers of iodine today are Chile and Japan. Due to its high atomic number and ease of attachment to [[organic compound]]s, it has also found favour as a non-toxic [[Radiocontrast agent|radiocontrast]] material. Because of the specificity of its uptake by the human body, radioactive isotopes of iodine can also be used to treat [[thyroid cancer]]. Iodine is also used as a [[Catalysis|catalyst]] in the industrial production of [[acetic acid]] and some [[polymer]]s.

It is on the [[WHO Model List of Essential Medicines|World Health Organization's List of Essential Medicines]].<ref name="WHO22nd">{{cite book | vauthors = ((World Health Organization)) | title = World Health Organization model list of essential medicines: 22nd list (2021) | year = 2021 | hdl = 10665/345533 | author-link = World Health Organization | publisher = World Health Organization | location = Geneva | id = WHO/MHP/HPS/EML/2021.02 | hdl-access=free }}</ref>

==History==
[[File:Iodine-evaporating.jpg|left|thumb|Iodine crystals [[Sublimation (phase transition)|sublimating]] into a purple gas]]
In 1811, iodine was discovered by French chemist [[Bernard Courtois]],<ref name="court">{{cite journal| vauthors = Courtois B  |title=Découverte d'une substance nouvelle dans le Vareck |trans-title=Discovery of a new substance in seaweed |journal=[[Annales de chimie]] |volume=88 |pages=304–310 |date=1813 |url=https://books.google.com/books?id=YGwri-w7sMAC&pg=RA2-PA304|language=French}} In French, seaweed that had been washed onto the shore was called "varec", "varech", or "vareck", whence the English word "wrack". Later, "varec" also referred to the ashes of such seaweed: the ashes were used as a source of iodine and salts of sodium and potassium.</ref><ref>{{cite journal | vauthors = Swain PA |title=Bernard Courtois (1777–1838) famed for discovering iodine (1811), and his life in Paris from 1798 |journal=Bulletin for the History of Chemistry |volume=30 |issue=2 |page=103 |date=2005 |doi=10.70359/bhc2005v030p103 |url=http://www.scs.uiuc.edu/~mainzv/HIST/awards/OPA%20Papers/2007-Swain.pdf |access-date=2 April 2009 |archive-url=https://web.archive.org/web/20100714110757/http://www.scs.uiuc.edu/~mainzv/HIST/awards/OPA%20Papers/2007-Swain.pdf |archive-date=14 July 2010 }}</ref> who was born to a family of manufacturers of [[potassium nitrate|saltpetre]] (an essential component of [[gunpowder]]). At the time of the [[Napoleonic Wars]], saltpetre was in great demand in [[France]]. Saltpetre produced from French [[Niter|nitre beds]] required [[sodium carbonate]], which could be isolated from [[seaweed]] collected on the coasts of [[Normandy]] and [[Brittany]]. To isolate the sodium carbonate, seaweed was burned and the ashes washed with water. While investigating the cause of corrosion to the copper vessels used in the process,<ref>{{Cite journal |date=1814-12-31 |title=VI. Some experiments and observations on a new substance which becomes a violet coloured gas by heat |url=https://royalsocietypublishing.org/doi/10.1098/rstl.1814.0007 |journal=Philosophical Transactions of the Royal Society of London |language=en |volume=104 |pages=74–93 |doi=10.1098/rstl.1814.0007 |issn=0261-0523|url-access=subscription }}</ref> Courtois added an excess of sulfuric acid to the waste remaining and a cloud of violet vapour arose. He noted that the vapour crystallised on cold surfaces, forming dark crystals.<ref name="Greenwood794">Greenwood and Earnshaw, p. 794</ref> Courtois suspected that this material was a new element but lacked funding to pursue it further.<ref name="vdK">{{cite web |url=http://elements.vanderkrogt.net/element.php?sym=i |title=53 Iodine |publisher=Elements.vanderkrogt.net |access-date=23 October 2016 |archive-date=23 January 2010 |archive-url=https://web.archive.org/web/20100123001444/http://elements.vanderkrogt.net/element.php?sym=I |url-status=live }}</ref>

Courtois gave samples to his friends, [[Charles Bernard Desormes]] (1777–1838) and [[Nicolas Clément]] (1779–1841), to continue research. He also gave some of the substance to chemist [[Joseph Louis Gay-Lussac]] (1778–1850), and to physicist [[André-Marie Ampère]] (1775–1836). On 29 November 1813, Desormes and Clément made Courtois' discovery public by describing the substance to a meeting of the Imperial [[Institut de France]].<ref>Desormes and Clément made their announcement at the Institut impérial de France on 29 November 1813; a summary of their announcement appeared in the ''Gazette nationale ou Le Moniteur Universel'' of 2 December 1813. See: 
*  {{cite journal |last1=(Staff) |title=Institut Imperial de France |journal=Le Moniteur Universel |date=2 December 1813 |issue=336 |page=1344 |url=https://www.retronews.fr/journal/gazette-nationale-ou-le-moniteur-universel/02-decembre-1813/149/1332251/2 |language=French |access-date=2 May 2021 |archive-date=28 November 2022 |archive-url=https://web.archive.org/web/20221128171041/https://www.retronews.fr/journal/gazette-nationale-ou-le-moniteur-universel/02-decembre-1813/149/1332251/2 |url-status=live }}
*  {{cite journal |vauthors=Chattaway FD |title=The discovery of iodine |journal=Chemical News and Journal of Industrial Science |date=23 April 1909 |volume=99 |issue=2578 |pages=193–195 |url=https://books.google.com/books?id=Rco_AQAAIAAJ&pg=PA193 }}</ref> On 6 December 1813, Gay-Lussac found and announced that the new substance was either an element or a compound of [[oxygen]] and he found that it is an element.<ref name="Gay-Lussac">{{cite journal |vauthors=Gay-Lussac J |title=Sur un nouvel acide formé avec la substance décourverte par M. Courtois |trans-title=On a new acid formed by the substance discovered by Mr. Courtois |journal=Annales de Chimie |volume=88 |pages=311–318 |date=1813 |url=https://books.google.com/books?id=YGwri-w7sMAC&pg=PA311 |language=French |access-date=2 May 2021 |archive-date=19 March 2024 |archive-url=https://web.archive.org/web/20240319070023/https://books.google.com/books?id=YGwri-w7sMAC&pg=PA311#v=onepage&q&f=false |url-status=live }}</ref><ref>{{cite journal |vauthors=Gay-Lussac J |title=Sur la combination de l'iode avec d'oxigène |trans-title=On the combination of iodine with oxygen |journal=Annales de Chimie |volume=88 |pages=319–321 |date=1813 |url=https://books.google.com/books?id=YGwri-w7sMAC&pg=PA319 |language=French |access-date=2 May 2021 |archive-date=19 March 2024 |archive-url=https://web.archive.org/web/20240319070022/https://books.google.com/books?id=YGwri-w7sMAC&pg=PA319#v=onepage&q&f=false |url-status=live }}</ref><ref>{{cite journal| vauthors = Gay-Lussac J |title=Mémoire sur l'iode |trans-title=Memoir on iodine |journal=Annales de Chimie |volume=91 |pages=5–160|date=1814 |url=https://books.google.com/books?id=Efms0Fri1CQC&pg=PA5|language=French}}</ref> Gay-Lussac suggested the name "iode" ([[Anglicisation (linguistics)|anglicised]] as "iodine"), from the [[Ancient Greek]] {{lang|grc|Ιώδης}} ({{transliteration|grc|iodēs}}, "violet"), because of the colour of iodine vapour.<ref name="court" /><ref name="Gay-Lussac" /> Ampère had given some of his sample to British chemist [[Humphry Davy]] (1778–1829), who experimented on the substance and noted its similarity to [[chlorine]] and also found it as an element.<ref>{{cite journal |vauthors=Davy H |author-link=Humphry Davy |title=Sur la nouvelle substance découverte par M. Courtois, dans le sel de Vareck |trans-title=On the new substance discovered by Mr. Courtois in the salt of seaweed |journal=Annales de Chimie |volume=88 |pages=322–329 |date=1813 |url=https://books.google.com/books?id=YGwri-w7sMAC&pg=PA322 |language=French |access-date=2 May 2021 |archive-date=19 March 2024 |archive-url=https://web.archive.org/web/20240319070024/https://books.google.com/books?id=YGwri-w7sMAC&pg=PA322#v=onepage&q&f=false |url-status=live }}</ref> Davy sent a letter dated 10 December to the [[Royal Society]] stating that he had identified a new element called iodine.<ref>{{cite journal| vauthors = Davy H |author-link=Humphry Davy |title=Some experiments and observations on a new substance which becomes a violet coloured gas by heat |journal=Philosophical Transactions of the Royal Society of London |volume=104 |pages=74–93 |date=1 January 1814 |doi=10.1098/rstl.1814.0007 |doi-access=free }}</ref> Arguments erupted between Davy and Gay-Lussac over who identified iodine first, but both scientists found that both of them identified iodine first and also knew that Courtois is the first one to isolate the element.<ref name="vdK" />

In 1873, the French medical researcher [[Casimir Davaine]] (1812–1882) discovered the antiseptic action of iodine.<ref>{{cite journal |vauthors=Davaine C |title=Recherches relatives à l'action des substances dites ''antiseptiques'' sur le virus charbonneux |journal=Comptes rendus hebdomadaires des séances de l'Académie des Sciences |date=1873 |volume=77 |pages=821–825 |url=https://babel.hathitrust.org/cgi/pt?id=uiug.30112025711521&view=1up&seq=829 |trans-title=Investigations regarding the action of so-called ''antiseptic'' substances on the anthrax bacterium |language=French |access-date=2 May 2021 |archive-date=5 May 2021 |archive-url=https://web.archive.org/web/20210505013431/https://babel.hathitrust.org/cgi/pt?id=uiug.30112025711521&view=1up&seq=829 |url-status=live }}</ref> [[Antonio Grossich]] (1849–1926), an Istrian-born surgeon, was among the first to use [[Sterilization (microbiology)|sterilisation]] of the operative field. In 1908, he introduced tincture of iodine as a way to rapidly sterilise the human skin in the surgical field.<ref>{{cite journal |vauthors=Grossich A |title=Eine neue Sterilisierungsmethode der Haut bei Operationen |journal=Zentralblatt für Chirurgie |date=31 October 1908 |volume=35 |issue=44 |pages=1289–1292 |url=https://babel.hathitrust.org/cgi/pt?id=uc1.b4150494&view=1up&seq=1305 |trans-title=A new method of sterilization of the skin for operations |language=German |access-date=2 May 2021 |archive-date=5 May 2021 |archive-url=https://web.archive.org/web/20210505130854/https://babel.hathitrust.org/cgi/pt?id=uc1.b4150494 |url-status=live }}</ref>

In early [[periodic table]]s, iodine was often given the symbol ''J'', for ''Jod'', its name in [[German language|German]]; in German texts, ''J'' is still frequently used in place of ''I''.<ref>{{cite web |title=Mendeleev's First Periodic Table |url=https://web.lemoyne.edu/giunta/EA/MENDELEEVann.HTML |website=web.lemoyne.edu |access-date=25 January 2019 |archive-date=10 May 2021 |archive-url=https://web.archive.org/web/20210510014806/https://web.lemoyne.edu/GIUNTA/EA/MENDELEEVann.HTML |url-status=live }}</ref>

==Properties==
[[File:IodoAtomico.JPG|thumb|left|upright=0.7|alt=Round bottom flask filled with violet iodine vapour|Iodine vapour in a flask, demonstrating its characteristic rich purple colour]]
Iodine is the fourth [[halogen]], being a member of group 17 in the periodic table, below [[fluorine]], [[chlorine]], and [[bromine]]; since [[astatine]] and [[tennessine]] are radioactive, iodine is the heaviest stable halogen. Iodine has an electron configuration of [Kr]5s<sup>2</sup>4d<sup>10</sup>5p<sup>5</sup>, with the seven electrons in the fifth and outermost shell being its [[valence electron]]s. Like the other halogens, it is one electron short of a full octet and is hence an oxidising agent, reacting with many elements in order to complete its outer shell, although in keeping with [[periodic trends]], it is the weakest oxidising agent among the stable halogens: it has the lowest [[electronegativity]] among them, just 2.66 on the Pauling scale (compare fluorine, chlorine, and bromine at 3.98, 3.16, and 2.96 respectively; astatine continues the trend with an electronegativity of 2.2). Elemental iodine hence forms [[diatomic molecule]]s with chemical formula I<sub>2</sub>, where two iodine atoms share a pair of electrons in order to each achieve a stable octet for themselves; at high temperatures, these diatomic molecules reversibly dissociate a pair of iodine atoms. Similarly, the iodide anion, I<sup>−</sup>, is the strongest reducing agent among the stable halogens, being the most easily oxidised back to diatomic I<sub>2</sub>.<ref name="Greenwood800">Greenwood and Earnshaw, pp. 800–4</ref> (Astatine goes further, being indeed unstable as At<sup>−</sup> and readily oxidised to At<sup>0</sup> or At<sup>+</sup>.)<ref>{{cite book | series = Gmelin Handbook of Inorganic and Organometallic Chemistry | title = 'At, Astatine', System No. 8a | edition=8th | year = 1985 | publisher = Springer-Verlag | isbn = 978-3-540-93516-2 | vauthors = Kugler HK, Keller C | volume = 8 }}</ref>

The halogens darken in colour as the group is descended: fluorine is a very pale yellow, chlorine is greenish-yellow, bromine is reddish-brown, and iodine is violet.

Elemental iodine is slightly soluble in water, with one gram dissolving in 3450&nbsp;mL at 20&nbsp;°C and 1280&nbsp;mL at 50&nbsp;°C; [[potassium iodide]] may be added to increase solubility via formation of [[triiodide]] ions, among other polyiodides.<ref name="Greenwood804">Greenwood and Earnshaw, pp. 804–9</ref> Nonpolar solvents such as [[hexane]] and [[carbon tetrachloride]] provide a higher solubility.<ref>{{cite book| title = Merck Index of Chemicals and Drugs| edition = 9th| date = 1976| isbn=978-0-911910-26-1| editor = Windholz, Martha| editor2 = Budavari, Susan| editor3 = Stroumtsos, Lorraine Y.| editor4 = Fertig, Margaret Noether| publisher = J A Majors Company}}</ref> Polar solutions, such as aqueous solutions, are brown, reflecting the role of these solvents as [[Lewis acids and bases|Lewis bases]]; on the other hand, nonpolar solutions are violet, the colour of iodine vapour.<ref name="Greenwood804" /> [[Charge-transfer complex]]es form when iodine is dissolved in polar solvents, hence changing the colour. Iodine is violet when dissolved in carbon tetrachloride and saturated hydrocarbons but deep brown in [[Alcohol (chemistry)|alcohol]]s and [[amine]]s, solvents that form charge-transfer adducts.<ref name="King">{{cite book | vauthors = King RB |date=1995 |title=Inorganic Chemistry of Main Group Elements |publisher=Wiley-VCH |pages=173–98|isbn=978-0-471-18602-1}}</ref>

The density, and the melting and boiling points of iodine follow the trend across all of the halogens of increasing with atomic number.<ref>{{Cite book |last=Sicius |first=Hermann |url=https://link.springer.com/10.1007/978-3-662-68921-9 |title=Handbook of the Chemical Elements |date=2024 |publisher=Springer Berlin Heidelberg |isbn=978-3-662-68920-2 |location=Berlin, Heidelberg |language=en |doi=10.1007/978-3-662-68921-9}}</ref>{{rp|405}} Among the stable halogens, iodine has the largest electron cloud among them that is the most easily polarised, resulting in its molecules having the strongest [[Van der Waals force|Van der Waals interactions]] among the stable halogens. Similarly, iodine is the least volatile of the stable halogens, though the solid still can be observed to give off purple vapour.<ref name="Greenwood800" /> Due to this property iodine is commonly used to demonstrate [[sublimation (phase transition)|sublimation]] directly from [[solid]] to [[gas]], which gives rise to a misconception that it does not [[melting|melt]] in [[atmospheric pressure]].<ref>{{cite journal |title=The concept of sublimation – iodine as an example |journal=Educación Química |date=1 March 2012 |volume=23 |pages=171–175 |doi=10.1016/S0187-893X(17)30149-0 |language=en |issn=0187-893X|doi-access=free | vauthors = Stojanovska M, Petruševski VM, Šoptrajanov B }}</ref> Because it has the largest [[atomic radius]] among the stable halogens, iodine has the lowest first [[Ionization energy|ionisation energy]], lowest [[electron affinity]], lowest [[electronegativity]] and lowest reactivity of the stable halogens.<ref name="Greenwood800" />

[[File:Iodine-unit-cell-3D-balls-B.png|thumb|upright=0.7|right|Structure of solid iodine]]
The interhalogen bond in diiodine is the weakest of all the stable halogens. As such, 1% of a sample of gaseous iodine at atmospheric pressure is dissociated into iodine atoms at 575&nbsp;°C. Temperatures greater than 750&nbsp;°C are required for fluorine, chlorine, and bromine to dissociate to a similar extent. Most bonds to iodine are weaker than the analogous bonds to the lighter halogens.<ref name="Greenwood800" /> Gaseous iodine is composed of I<sub>2</sub> molecules with an I–I bond length of 266.6&nbsp;pm. The I–I bond is one of the longest single bonds known. It is even longer (271.5&nbsp;pm) in solid [[Orthorhombic crystal system|orthorhombic]] crystalline iodine, which has the same crystal structure as chlorine and bromine. (The record is held by iodine's neighbour [[xenon]]: the Xe–Xe bond length is 308.71&nbsp;pm.)<ref>{{cite book| title = Advanced Structural Inorganic Chemistry| url = https://archive.org/details/advancedstructur00liwa| url-access = limited| vauthors = Li WK, Zhou GD, Mak TC | publisher = Oxford University Press| date = 2008| isbn = 978-0-19-921694-9| page = [https://archive.org/details/advancedstructur00liwa/page/n696 674]}}</ref> As such, within the iodine molecule, significant electronic interactions occur with the two next-nearest neighbours of each atom, and these interactions give rise, in bulk iodine, to a shiny appearance and [[semiconductor|semiconducting]] properties.<ref name="Greenwood800" /> Iodine is a two-dimensional semiconductor with a [[band gap]] of 1.3&nbsp;eV (125&nbsp;kJ/mol): it is a semiconductor in the plane of its crystalline layers and an insulator in the perpendicular direction.<ref name="Greenwood800" />

===Isotopes===
{{main|Isotopes of iodine}}
Naturally occurring iodine consists of one stable isotope, <sup>127</sup>I, and is a [[mononuclidic element|mononuclidic]] element for atomic weight, which is thus a constant of nature determined by that isotope.<ref name="CIAAWiodine" /> Radioisotopes are known from <sup>108</sup>I to <sup>147</sup>I. As other isotopes have half-lives too short to be [[primordial nuclide|primordial]], it is also [[monoisotopic element|monoisotopic]].

The longest-lived of the radioactive isotopes of iodine is [[iodine-129]], which has a half-life of 16.1&nbsp;million&nbsp;years, decaying via [[beta decay]] to stable [[xenon]]-129.<ref name="NUBASE">{{NUBASE2020}}</ref> Some iodine-129 was formed along with iodine-127 before the formation of the [[Solar System]], but it has by now completely decayed away, making it an [[extinct radionuclide]]. Its former presence may be determined from an excess of its [[decay product|daughter]] xenon-129, but early attempts<ref name="Reynolds1960a">{{Cite journal |last=Reynolds |first=J. H. |date=1 January 1960 |title=Determination of the Age of the Elements |url=https://link.aps.org/doi/10.1103/PhysRevLett.4.8 |journal=Physical Review Letters |language=en |volume=4 |issue=1 |pages=8–10 |doi=10.1103/PhysRevLett.4.8 |bibcode=1960PhRvL...4....8R |issn=0031-9007|url-access=subscription }}</ref> to use this characteristic to date the supernova source for elements in the Solar System are made difficult by alternative nuclear processes giving iodine-129 and by iodine's volatility at higher temperatures.<ref name="Manuel2002">{{Cite book |last=Manuel |first=O. |date=2002 |editor-last=Manuel |editor-first=O. |chapter=Origin of Elements in the Solar System |title=Origin of Elements in the Solar System<!--yes, the chapter and the book have the same title--> |chapter-url=http://link.springer.com/10.1007/0-306-46927-8_44 |language=en |location=Boston, MA |publisher=Springer US |pages=589–643 |doi=10.1007/0-306-46927-8_44 |isbn=978-0-306-46562-8}}</ref> Due to its mobility in the environment iodine-129 has been used to date very old groundwaters.<ref>{{cite journal | vauthors = Watson JT, Roe DK, Selenkow HA | title = Iodine-129 as a "nonradioactive" tracer | journal = Radiation Research | volume = 26 | issue = 1 | pages = 159–163 | date = September 1965 | pmid = 4157487 | doi = 10.2307/3571805 | bibcode = 1965RadR...26..159W | jstor = 3571805 }}</ref><ref>{{cite journal | vauthors = Snyder G, Fabryka-Martin J | year = 2007 | title = I-129 and Cl-36 in dilute hydrocarbon waters: Marine-cosmogenic, in situ, and anthropogenic sources | journal = Applied Geochemistry | volume = 22 | issue = 3| pages = 692–714 | doi = 10.1016/j.apgeochem.2006.12.011 | bibcode = 2007ApGC...22..692S }}</ref> 

The vast majority of iodine-129 on Earth today derives from human nuclear activity. Iodine-129 increased 3-8 orders of magnitude after nuclear activity began.<ref>{{Cite journal |last1=Hou |first1=Xiaolin |last2=Hansen |first2=Violeta |last3=Aldahan |first3=Ala |last4=Possnert |first4=Göran |last5=Lind |first5=Ole Christian |last6=Lujaniene |first6=Galina |date=2009-01-26 |title=A review on speciation of iodine-129 in the environmental and biological samples |url=https://www.sciencedirect.com/science/article/pii/S0003267008019211 |journal=Analytica Chimica Acta |volume=632 |issue=2 |pages=181–196 |doi=10.1016/j.aca.2008.11.013 |pmid=19110092 |bibcode=2009AcAC..632..181H |issn=0003-2670}}</ref>
A small amount of naturally occurring iodine-129 forms from [[cosmic ray spallation]] of atmospheric xenon and as a fission product; the ratio <sup>129</sup>I/<sup>127</sup>I is about 10<sup>−12</sup>.<ref>Szidat, S., Michel, R., Handl, J., Jakob, D., Synal, H. A., & Suter, M. (2000). Status and trends of iodine-129 abundances in the European environment. Hiroshima (Japan).[https://www.irpa.net/irpa10/cdrom/00084.pdf Status and Trends of Iodine-129 Abundances in the European Environment]</ref> 

Excited states of iodine-127 and iodine-129 are often used in [[Mössbauer spectroscopy]].<ref name="Greenwood800" />

The other iodine radioisotopes have much shorter half-lives, less than 60 days.<ref name="NUBASE" /> Some of them have medical applications involving the [[Thyroid|thyroid gland]], where the iodine that enters the body is stored and concentrated. [[Iodine-123]] (half-life 13.223 hours) and decays by [[electron capture]] to [[Isotopes of tellurium|tellurium-123]], emitting [[Gamma ray|gamma radiation]]; it is used in [[Nuclear medicine|nuclear medicine imaging]], including [[Single-photon emission computed tomography|single photon emission computed tomography]] (SPECT) and [[CT scan|X-ray computed tomography]] (X-Ray CT) scans.<ref>{{cite journal | vauthors = Hupf HB, Eldridge JS, Beaver JE | title = Production of iodine-123 for medical applications | journal = The International Journal of Applied Radiation and Isotopes | volume = 19 | issue = 4 | pages = 345–351 | date = April 1968 | pmid = 5650883 | doi = 10.1016/0020-708X(68)90178-6 }}</ref> [[Iodine-125]] (half-life 59.392 days) is similar, decaying by electron capture to [[Isotopes of tellurium|tellurium-125]] and emitting low-energy gamma radiation; the second-longest-lived iodine radioisotope, it has uses in [[Assay|biological assays]], [[nuclear medicine|nuclear medicine imaging]] and in [[radiation therapy]] as [[brachytherapy]] to treat a number of conditions, including [[prostate cancer]], [[uveal melanoma]]s, and [[Brain tumor|brain tumours]].<ref>Harper, P.V.; Siemens, W.D.; Lathrop, K.A.; Brizel, H.E.; Harrison, R.W. ''Iodine-125.'' Proc. Japan Conf. Radioisotopes; Vol: 4 January 1, 1961</ref> Finally, [[iodine-131]] (half-life 8.0249 days) beta-decays to [[Isotopes of xenon|xenon-131]] and also emits gamma radiation. It is also be used for medicinal purposes in [[radiation therapy]] to the thyroid, when tissue destruction is desired after iodine uptake by the tissue.<ref>{{cite news|title=Medical isotopes the likely cause of radiation in Ottawa waste|url=http://www.cbc.ca/news/canada/medical-isotopes-the-likely-cause-of-radiation-in-ottawa-waste-1.852645|date=4 February 2009|publisher=[[CBC News]]|access-date=30 September 2015|archive-date=19 November 2021|archive-url=https://web.archive.org/web/20211119213013/https://www.cbc.ca/news/canada/medical-isotopes-the-likely-cause-of-radiation-in-ottawa-waste-1.852645|url-status=live}}</ref>

Iodine-131 is a common [[Nuclear fission product|fission product]] and thus is present in high levels in radioactive [[Nuclear fallout|fallout]]. It may then be absorbed through contaminated food, and will also accumulate in the thyroid and damage it through its radiation. The primary risk from exposure to high levels of iodine-131 is the chance occurrence of [[Radiogenic nuclide|radiogenic]] [[thyroid cancer]] in later life. Other risks include the possibility of non-cancerous growths and [[thyroiditis]].<ref name="Rivkees">{{cite journal | vauthors = Rivkees SA, Sklar C, Freemark M | title = Clinical review 99: The management of Graves' disease in children, with special emphasis on radioiodine treatment | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 83 | issue = 11 | pages = 3767–3776 | date = November 1998 | pmid = 9814445 | doi = 10.1210/jcem.83.11.5239 | doi-access = free }}</ref> Protection against the negative effects of iodine-131 upon a release is effected by saturating the thyroid gland with stable iodine-127 in the form of [[potassium iodide]] tablets, taken daily for optimal prophylaxis.<ref>{{cite journal | vauthors = Zanzonico PB, Becker DV | title = Effects of time of administration and dietary iodine levels on potassium iodide (KI) blockade of thyroid irradiation by 131I from radioactive fallout | journal = Health Physics | volume = 78 | issue = 6 | pages = 660–667 | date = June 2000 | pmid = 10832925 | doi = 10.1097/00004032-200006000-00008 | bibcode = 2000HeaPh..78..660Z | s2cid = 30989865 }}</ref>

Iodine-131 has also been used as a [[radioactive tracer]].<ref>{{cite book|vauthors=Moser H, Rauert W|title=Isotopes in the water cycle: past, present and future of a developing science|year=2007|publisher=Springer|location=Dordrecht|isbn=978-1-4020-6671-9|veditors=Aggarwal PK, Gat JR, Froehlich KF|access-date=6 May 2012|page=11|chapter=Isotopic Tracers for Obtaining Hydrologic Parameters|chapter-url=https://books.google.com/books?id=XKk6V_IeJbIC&pg=PA11|archive-date=19 March 2024|archive-url=https://web.archive.org/web/20240319070244/https://books.google.com/books?id=XKk6V_IeJbIC&pg=PA11#v=onepage&q&f=false|url-status=live}}</ref><ref>{{cite book|vauthors=Rao SM|title=Practical isotope hydrology|year=2006|publisher=New India Publishing Agency|location=New Delhi|isbn=978-81-89422-33-2|chapter-url=https://books.google.com/books?id=E7TVDVVji0EC&q=isotope%20hydrology%20iodine&pg=PA11|access-date=6 May 2012|pages=12–13|chapter=Radioisotopes of hydrological interest|archive-date=19 March 2024|archive-url=https://web.archive.org/web/20240319070414/https://books.google.com/books?id=E7TVDVVji0EC&q=isotope%20hydrology%20iodine&pg=PA11|url-status=live}}</ref><ref>{{cite web|title=Investigating leaks in Dams & Reservoirs|url=http://www.iaea.org/technicalcooperation/documents/sheet20dr.pdf|work=IAEA.org|access-date=6 May 2012|archive-url=https://web.archive.org/web/20130730053205/http://www.iaea.org/technicalcooperation/documents/sheet20dr.pdf|archive-date=30 July 2013}}</ref><ref>{{cite book|vauthors=Araguás LA, Bedmar AP|title=Detection and prevention of leaks from dams|year=2002|publisher=Taylor & Francis|isbn=978-90-5809-355-4|chapter-url=https://books.google.com/books?id=FXB-HMzfBnkC&pg=PA179|access-date=6 May 2012|pages=179–181|chapter=Artificial radioactive tracers|archive-date=19 March 2024|archive-url=https://web.archive.org/web/20240319070244/https://books.google.com/books?id=FXB-HMzfBnkC&pg=PA179#v=onepage&q&f=false|url-status=live}}</ref>

== Chemistry and compounds ==
{{Main|Iodine compounds}}
{| class="wikitable" style="float:right; width:25%;"
|+ style="margin-bottom: 5px;" | Halogen bond energies (kJ/mol)<ref name="Greenwood804" />
|-
! X
! XX
! HX
! BX<sub>3</sub>
! AlX<sub>3</sub>
! CX<sub>4</sub>
|-
! F
| 159
| 574
| 645
| 582
| 456
|-
! Cl
|243
|428
|444
|427
|327
|-
! Br
|193
|363
|368
|360
|272
|-
! I
|151
|294
|272
|285
|239
|}
Iodine is quite reactive, but it is less so than the lighter halogens, and it is a weaker oxidant. For example, it does not [[Halogenation|halogenate]] [[carbon monoxide]], [[nitric oxide]], and [[sulfur dioxide]], which [[chlorine]] does. Many metals react with iodine.<ref name="Greenwood800" /> For the same reason, however, since iodine has the lowest ionisation energy among the halogens and is the most easily oxidised of them, it has a more significant cationic chemistry and its higher oxidation states are rather more stable than those of bromine and chlorine, for example in [[iodine heptafluoride]].<ref name="Greenwood804" />

===Charge-transfer complexes ===
[[File:Iodine-triphenylphosphine charge-transfer complex in dichloromethane.jpg|thumb|upright=1.8|right|I<sub>2</sub>•[[triphenylphosphine|PPh<sub>3</sub>]] charge-transfer complexes in [[dichloromethane|CH<sub>2</sub>Cl<sub>2</sub>]]. From left to right: (1) I<sub>2</sub> dissolved in dichloromethane – no CT complex. (2) A few seconds after excess PPh<sub>3</sub> was added – CT complex is forming. (3) One minute later after excess PPh<sub>3</sub> was added, the CT complex [Ph<sub>3</sub>PI]<sup>+</sup>I<sup>−</sup> has been formed. (4) Immediately after excess I<sub>2</sub> was added, which contains [Ph<sub>3</sub>PI]<sup>+</sup>[I<sub>3</sub>]<sup>−</sup>.<ref name="InorgChem">{{Housecroft3rd|page=541}}</ref>]]

The iodine molecule, I<sub>2</sub>, dissolves in CCl<sub>4</sub> and aliphatic hydrocarbons to give bright violet solutions. In these solvents the absorption band maximum occurs in the 520 &ndash; 540&nbsp;nm region and is assigned to a {{pi}}<sup>*</sup> to ''σ''<sup>*</sup> transition.   When I<sub>2</sub> reacts with Lewis bases in these solvents a blue shift in I<sub>2</sub> peak is seen and the new peak (230 &ndash; 330&nbsp;nm) arises that is due to the formation of adducts, which are referred to as charge-transfer complexes.<ref name="Greenwood806">Greenwood and Earnshaw, pp. 806–07</ref>

===Hydrogen iodide===
The simplest compound of iodine is [[hydrogen iodide]], HI. It is a colourless gas that reacts with oxygen to give water and iodine. Although it is useful in [[Halogenation|iodination]] reactions in the laboratory, it does not have large-scale industrial uses, unlike the other hydrogen halides. Commercially, it is usually made by reacting iodine with [[hydrogen sulfide]] or [[hydrazine]]:<ref name="Greenwood809">Greenwood and Earnshaw, pp. 809–812</ref>

:2 I<sub>2</sub> + N<sub>2</sub>H<sub>4</sub> {{overset|H<sub>2</sub>O|⟶}} 4 HI + N<sub>2</sub>

At room temperature, it is a colourless gas, like all of the hydrogen halides except [[hydrogen fluoride]], since hydrogen cannot form strong [[hydrogen bond]]s to the large and only mildly electronegative iodine atom. It melts at {{convert|−51.0|°C}} and boils at {{convert|−35.1|°C}}. It is an [[Endothermic process|endothermic]] compound that can exothermically dissociate at room temperature, although the process is very slow unless a [[Catalysis|catalyst]] is present: the reaction between hydrogen and iodine at room temperature to give hydrogen iodide does not proceed to completion. The H–I [[Bond-dissociation energy|bond dissociation energy]] is likewise the smallest of the hydrogen halides, at 295&nbsp;kJ/mol.<ref name="Greenwood812">Greenwood and Earnshaw, pp. 812–819</ref>

Aqueous hydrogen iodide is known as [[hydroiodic acid]], which is a strong acid. Hydrogen iodide is exceptionally soluble in water: one litre of water will dissolve 425 litres of hydrogen iodide, and the saturated solution has only four water molecules per molecule of hydrogen iodide.<ref>{{Cite book |last1=Holleman |first1=A. F. |title=Inorganic Chemistry |last2=Wiberg |first2=E. |publisher=Academic Press |year=2001 |isbn=0-12-352651-5 |location=San Diego}}</ref> Commercial so-called "concentrated" hydroiodic acid usually contains 48–57% HI by mass; the solution forms an [[azeotrope]] with boiling point {{convert|126.7|°C}} at 56.7&nbsp;g HI per 100&nbsp;g solution. Hence hydroiodic acid cannot be concentrated past this point by evaporation of water.<ref name="Greenwood812" /> Unlike gaseous hydrogen iodide, hydroiodic acid has major industrial use in the manufacture of [[acetic acid]] by the [[Cativa process]].<ref name="Cativa">{{Cite journal |last=Jones |first=J. H. |year=2000 |title=The Cativa Process for the Manufacture of Acetic Acid |url=http://www.platinummetalsreview.com/pdf/pmr-v44-i3-094-105.pdf |url-status=live |journal=[[Platinum Metals Review]] |volume=44 |issue=3 |pages=94–105 |doi=10.1595/003214000X44394105 |archive-url=https://web.archive.org/web/20150924074441/http://www.platinummetalsreview.com/pdf/pmr-v44-i3-094-105.pdf |archive-date=24 September 2015 |access-date=26 August 2023}}</ref><ref>{{Cite journal |last1=Sunley |first1=G. J. |last2=Watsonv |first2=D. J. |year=2000 |title=High productivity methanol carbonylation catalysis using iridium – The Cativa process for the manufacture of acetic acid |journal=Catalysis Today |volume=58 |issue=4 |pages=293–307 |doi=10.1016/S0920-5861(00)00263-7}}</ref>

===Other binary iodine compounds===
With the exception of the [[Noble gas|noble gases]], nearly all elements on the periodic table up to einsteinium ([[Einsteinium(III) iodide|EsI<sub>3</sub>]] is known) are known to form binary compounds with iodine. Until 1990, [[nitrogen triiodide]]<ref>The ammonia adduct NI<sub>3</sub>•NH<sub>3</sub> is more stable and can be isolated at room temperature as a notoriously shock-sensitive black solid.</ref> was only known as an ammonia adduct. Ammonia-free NI<sub>3</sub> was found to be isolable at –196 °C but spontaneously decomposes at 0 °C.<ref>{{cite journal |last1=Tornieporth-Oetting |first1=Inis |last2=Klapötke |first2=Thomas |date=June 1990 |title=Nitrogen Triiodide |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.199006771 |journal=Angewandte Chemie |edition=international |language=en |volume=29 |issue=6 |pages=677–679 |doi=10.1002/anie.199006771 |issn=0570-0833 |access-date=5 March 2023 |archive-date=5 March 2023 |archive-url=https://web.archive.org/web/20230305194218/https://onlinelibrary.wiley.com/doi/10.1002/anie.199006771 |url-status=live |url-access=subscription }}</ref> For thermodynamic reasons related to electronegativity of the elements, neutral sulfur and selenium iodides that are stable at room temperature are also nonexistent, although S<sub>2</sub>I<sub>2</sub> and SI<sub>2</sub> are stable up to 183 and 9 K, respectively.  As of 2022, no neutral binary selenium iodide has been unambiguously identified (at any temperature).<ref>{{cite journal |last=Vilarrubias |first=Pere |date=17 November 2022 |title=The elusive diiodosulphanes and diiodoselenanes |journal=Molecular Physics |volume=120 |issue=22 |article-number=e2129106 |doi=10.1080/00268976.2022.2129106 |bibcode=2022MolPh.12029106V |s2cid=252744393 |issn=0026-8976 }}</ref> Sulfur-iodine and selenium-iodine polyatomic cations (e.g., [S<sub>2</sub>I<sub>4</sub><sup>2+</sup>][AsF<sub>6</sub><sup>–</sup>]<sub>2</sub> and [Se<sub>2</sub>I<sub>4</sub><sup>2+</sup>][Sb<sub>2</sub>F<sub>11</sub><sup>–</sup>]<sub>2</sub>) have been prepared and characterised crystallographically.<ref>{{cite journal |last1=Klapoetke |first1=T. |last2=Passmore |first2=J. |date=1 July 1989 |title=Sulfur and selenium iodine compounds: from non-existence to significance |url=https://pubs.acs.org/doi/abs/10.1021/ar00163a002 |journal=Accounts of Chemical Research |language=en |volume=22 |issue=7 |pages=234–240 |doi=10.1021/ar00163a002 |issn=0001-4842 |access-date=15 January 2023 |archive-date=15 January 2023 |archive-url=https://web.archive.org/web/20230115160630/https://pubs.acs.org/doi/abs/10.1021/ar00163a002 |url-status=live |url-access=subscription }}</ref>

Given the large size of the iodide anion and iodine's weak oxidising power, high oxidation states are difficult to achieve in binary iodides, the maximum known being in the pentaiodides of [[niobium]], [[tantalum]], and [[protactinium]]. Iodides can be made by reaction of an element or its oxide, hydroxide, or carbonate with hydroiodic acid, and then dehydrated by mildly high temperatures combined with either low pressure or anhydrous hydrogen iodide gas. These methods work best when the iodide product is stable to hydrolysis. Other syntheses include high-temperature oxidative iodination of the element with iodine or hydrogen iodide, high-temperature iodination of a metal oxide or other halide by iodine, a volatile metal halide, [[carbon tetraiodide]], or an organic iodide. For example, [[Molybdenum dioxide|molybdenum(IV) oxide]] reacts with [[Aluminium iodide|aluminium(III) iodide]] at 230&nbsp;°C to give [[molybdenum(II) iodide]]. An example involving halogen exchange is given below, involving the reaction of [[tantalum(V) chloride]] with excess aluminium(III) iodide at 400&nbsp;°C to give [[tantalum(V) iodide]]:<ref name="Greenwood821">Greenwood and Earnshaw, pp. 821–4</ref>

<chem display="block">3TaCl5 + \underset{(excess)}{5AlI3} -> 3TaI5 + 5AlCl3</chem>

Lower iodides may be produced either through thermal decomposition or disproportionation, or by reducing the higher iodide with hydrogen or a metal, for example:<ref name="Greenwood821" />

<chem display="block">TaI5{} + Ta ->[\text{thermal gradient}] [\ce{630^\circ C\ ->\ 575^\circ C}] Ta6I14</chem>

Most metal iodides with the metal in low oxidation states (+1 to +3) are ionic. Nonmetals tend to form covalent molecular iodides, as do metals in high oxidation states from +3 and above. Both ionic and covalent iodides are known for metals in oxidation state +3 (e.g. [[Scandium triiodide|scandium iodide]] is mostly ionic, but [[aluminium iodide]] is not). Ionic iodides MI<sub>''n''</sub> tend to have the lowest melting and boiling points among the halides MX<sub>''n''</sub> of the same element, because the electrostatic forces of attraction between the cations and anions are weakest for the large iodide anion. In contrast, covalent iodides tend to instead have the highest melting and boiling points among the halides of the same element, since iodine is the most polarisable of the halogens and, having the most electrons among them, can contribute the most to van der Waals forces. Naturally, exceptions abound in intermediate iodides where one trend gives way to the other. Similarly, solubilities in water of predominantly ionic iodides (e.g. [[potassium]] and [[calcium]]) are the greatest among ionic halides of that element, while those of covalent iodides (e.g. [[silver]]) are the lowest of that element. In particular, [[silver iodide]] is very insoluble in water and its formation is often used as a qualitative test for iodine.<ref name="Greenwood821" />

===Iodine halides===
The halogens form many binary, [[Diamagnetism|diamagnetic]] [[interhalogen]] compounds with stoichiometries XY, XY<sub>3</sub>, XY<sub>5</sub>, and XY<sub>7</sub> (where X is heavier than Y), and iodine is no exception. Iodine forms all three possible diatomic interhalogens, a trifluoride and trichloride, as well as a pentafluoride and, exceptionally among the halogens, a heptafluoride. Numerous cationic and anionic derivatives are also characterised, such as the wine-red or bright orange compounds of {{chem|ICl|2|+}} and the dark brown or purplish black compounds of I<sub>2</sub>Cl<sup>+</sup>. Apart from these, some [[pseudohalogen|pseudohalides]] are also known, such as [[cyanogen iodide]] (ICN), iodine [[thiocyanate]] (ISCN), and iodine [[azide]] (IN<sub>3</sub>).<ref name="Greenwood824">Greenwood and Earnshaw, pp. 824–828</ref>

[[File:Iodine monochloride1.jpg|thumb|right|Iodine monochloride]]
[[Iodine monofluoride]] (IF) is unstable at room temperature and disproportionates very readily and irreversibly to iodine and [[iodine pentafluoride]], and thus cannot be obtained pure. It can be synthesised from the reaction of iodine with fluorine gas in [[trichlorofluoromethane]] at −45&nbsp;°C, with [[iodine trifluoride]] in trichlorofluoromethane at −78&nbsp;°C, or with [[silver(I) fluoride]] at 0&nbsp;°C.<ref name="Greenwood824" /> [[Iodine monochloride]] (ICl) and [[iodine monobromide]] (IBr), on the other hand, are moderately stable. The former, a volatile red-brown compound, was discovered independently by [[Joseph Louis Gay-Lussac]] and [[Humphry Davy]] in 1813–1814 not long after the discoveries of chlorine and iodine, and it mimics the intermediate halogen bromine so well that [[Justus von Liebig]] was misled into mistaking bromine (which he had found) for iodine monochloride. Iodine monochloride and iodine monobromide may be prepared simply by reacting iodine with chlorine or bromine at room temperature and purified by [[fractional crystallization (chemistry)|fractional crystallisation]]. Both are quite reactive and attack even [[platinum]] and [[gold]], though not [[boron]], [[carbon]], [[cadmium]], [[lead]], [[zirconium]], [[niobium]], [[molybdenum]], and [[tungsten]]. Their reaction with organic compounds depends on conditions. Iodine chloride vapour tends to chlorinate [[phenol]] and [[salicylic acid]], since when iodine chloride undergoes [[Homolysis (chemistry)|homolytic fission]], chlorine and iodine are produced and the former is more reactive. However, iodine chloride in [[carbon tetrachloride]] solution results in iodination being the main reaction, since now [[Heterolysis (chemistry)|heterolytic fission]] of the I–Cl bond occurs and I<sup>+</sup> attacks phenol as an electrophile. However, iodine monobromide tends to brominate phenol even in carbon tetrachloride solution because it tends to dissociate into its elements in solution, and bromine is more reactive than iodine.<ref name="Greenwood824" /> When liquid, iodine monochloride and iodine monobromide dissociate into {{chem|I|2|X|+}} and {{chem|IX|2|-}} ions (X = Cl, Br); thus they are significant conductors of electricity and can be used as ionising solvents.<ref name="Greenwood824" />

[[Iodine trifluoride]] (IF<sub>3</sub>) is an unstable yellow solid that decomposes above −28&nbsp;°C. It is thus little-known. It is difficult to produce because fluorine gas would tend to oxidise iodine all the way to the pentafluoride; reaction at low temperature with [[xenon difluoride]] is necessary. [[Iodine trichloride]], which exists in the solid state as the planar dimer I<sub>2</sub>Cl<sub>6</sub>, is a bright yellow solid, synthesised by reacting iodine with liquid chlorine at −80&nbsp;°C; caution is necessary during purification because it easily dissociates to iodine monochloride and chlorine and hence can act as a strong chlorinating agent. Liquid iodine trichloride conducts electricity, possibly indicating dissociation to {{chem|ICl|2|+}} and {{chem|ICl|4|-}} ions.<ref name="Greenwood828">Greenwood and Earnshaw, pp. 828–831</ref>

[[Iodine pentafluoride]] (IF<sub>5</sub>), a colourless, volatile liquid, is the most thermodynamically stable iodine fluoride, and can be made by reacting iodine with fluorine gas at room temperature. It is a fluorinating agent, but is mild enough to store in glass apparatus. Again, slight electrical conductivity is present in the liquid state because of dissociation to {{chem|IF|4|+}} and {{chem|IF|6|-}}. The [[pentagonal bipyramidal molecular geometry|pentagonal bipyramidal]] [[iodine heptafluoride]] (IF<sub>7</sub>) is an extremely powerful fluorinating agent, behind only [[chlorine trifluoride]], [[chlorine pentafluoride]], and [[bromine pentafluoride]] among the interhalogens: it reacts with almost all the elements even at low temperatures, fluorinates [[Pyrex]] glass to form iodine(VII) oxyfluoride (IOF<sub>5</sub>), and sets [[carbon monoxide]] on fire.<ref name="Greenwood832">Greenwood and Earnshaw, pp. 832–835</ref>

===Iodine oxides and oxoacids===
[[File:Iodine-pentoxide-3D-balls.png|thumb|right|upright=0.7|Structure of iodine pentoxide]]
[[Iodine oxide]]s are the most stable of all the halogen oxides, because of the strong I–O bonds resulting from the large electronegativity difference between iodine and oxygen, and they have been known for the longest time.<ref name="King" /> The stable, white, [[Hygroscopy|hygroscopic]] [[iodine pentoxide]] (I<sub>2</sub>O<sub>5</sub>) has been known since its formation in 1813 by Gay-Lussac and Davy. It is most easily made by the dehydration of [[iodic acid]] (HIO<sub>3</sub>), of which it is the anhydride. It will quickly oxidise carbon monoxide completely to [[carbon dioxide]] at room temperature, and is thus a useful reagent in determining carbon monoxide concentration. It also oxidises [[nitrogen oxide]], [[ethylene]], and [[hydrogen sulfide]]. It reacts with [[sulfur trioxide]] and peroxydisulfuryl difluoride (S<sub>2</sub>O<sub>6</sub>F<sub>2</sub>) to form salts of the iodyl cation, [IO<sub>2</sub>]<sup>+</sup>, and is reduced by concentrated [[sulfuric acid]] to iodosyl salts involving [IO]<sup>+</sup>. It may be fluorinated by [[fluorine]], [[bromine trifluoride]], [[sulfur tetrafluoride]], or [[chloryl fluoride]], resulting [[iodine pentafluoride]], which also reacts with [[iodine pentoxide]], giving iodine(V) oxyfluoride, IOF<sub>3</sub>. A few other less stable oxides are known, notably I<sub>4</sub>O<sub>9</sub> and I<sub>2</sub>O<sub>4</sub>; their structures have not been determined, but reasonable guesses are I<sup>III</sup>(I<sup>V</sup>O<sub>3</sub>)<sub>3</sub> and [IO]<sup>+</sup>[IO<sub>3</sub>]<sup>−</sup> respectively.<ref name="Greenwood851">Greenwood and Earnshaw, pp. 851–853</ref>

{| class="wikitable" style="float:right; width:25%;"
|+ Standard reduction potentials for aqueous I species<ref name="Greenwood853" />
! {{nowrap|E°(couple)}}!!{{nowrap|''a''(H<sup>+</sup>) {{=}} 1}}<br />(acid)!!{{nowrap|E°(couple)}}!!{{nowrap|''a''(OH<sup>−</sup>) {{=}} 1}}<br />(base)
|-
|I<sub>2</sub>/I<sup>−</sup>||+0.535|||I<sub>2</sub>/I<sup>−</sup>||+0.535
|-
|HOI/I<sup>−</sup>||+0.987||IO<sup>−</sup>/I<sup>−</sup>||+0.48
|-
|0||0||{{chem|IO|3|-}}/I<sup>−</sup>||+0.26
|-
|HOI/I<sub>2</sub>||+1.439||IO<sup>−</sup>/I<sub>2</sub>||+0.42
|-
|{{chem|IO|3|-}}/I<sub>2</sub>||+1.195||0||0
|-
|{{chem|IO|3|-}}/HOI||+1.134||{{chem|IO|3|-}}/IO<sup>−</sup>||+0.15
|-
|{{chem|IO|4|-}}/{{chem|IO|3|-}}||+1.653||0||0
|-
|H<sub>5</sub>IO<sub>6</sub>/{{chem|IO|3|-}}||+1.601||{{chem|H|3|IO|6|2-}}/{{chem|IO|3|-}}||+0.65
|}
More important are the four oxoacids: [[hypoiodous acid]] (HIO), [[Iodite|iodous acid]] (HIO<sub>2</sub>), [[iodic acid]] (HIO<sub>3</sub>), and [[periodic acid]] (HIO<sub>4</sub> or H<sub>5</sub>IO<sub>6</sub>). When iodine dissolves in aqueous solution, the following reactions occur:<ref name="Greenwood853">Greenwood and Earnshaw, pp. 853–9</ref>

{{block indent|{{wikitable|
|-
| I<sub>2</sub> + H<sub>2</sub>O || {{eqm}} HIO + H<sup>+</sup> + I<sup>−</sup> || ''K''<sub>ac</sub> = 2.0 × 10<sup>−13</sup> mol<sup>2</sup> L<sup>−2</sup>
|-
| I<sub>2</sub> + 2 OH<sup>−</sup> || {{eqm}} IO<sup>−</sup> + H<sub>2</sub>O + I<sup>−</sup> || ''K''<sub>alk</sub> {{=}} 30&nbsp;mol<sup>2</sup> L<sup>−2</sup>
}}}}

Hypoiodous acid is unstable to disproportionation. The hypoiodite ions thus formed disproportionate immediately to give iodide and iodate:<ref name="Greenwood853" />

{{block indent| 3 IO<sup>−</sup> {{eqm}} 2 I<sup>−</sup> + {{chem|IO|3|-}} ''K'' {{=}} 10<sup>20</sup>}}

Iodous acid and iodite are even less stable and exist only as a fleeting intermediate in the oxidation of iodide to iodate, if at all.<ref name="Greenwood853" /> Iodates are by far the most important of these compounds, which can be made by oxidising [[alkali metal]] iodides with oxygen at 600&nbsp;°C and high pressure, or by oxidising iodine with [[chlorate]]s. Unlike chlorates, which disproportionate very slowly to form chloride and perchlorate, iodates are stable to disproportionation in both acidic and alkaline solutions. From these, salts of most metals can be obtained. Iodic acid is most easily made by oxidation of an aqueous iodine suspension by [[electrolysis]] or fuming [[nitric acid]]. Iodate has the weakest oxidising power of the halates, but reacts the quickest.<ref name="Greenwood863">Greenwood and Earnshaw, pp. 863–4</ref>

Many periodates are known, including not only the expected tetrahedral {{chem|IO|4|-}}, but also square-pyramidal {{chem|IO|5|3-}}, octahedral orthoperiodate {{chem|IO|6|5-}}, [IO<sub>3</sub>(OH)<sub>3</sub>]<sup>2−</sup>, [I<sub>2</sub>O<sub>8</sub>(OH<sub>2</sub>)]<sup>4−</sup>, and {{chem|I|2|O|9|4-}}. They are usually made by oxidising alkaline [[sodium iodate]] electrochemically (with [[Lead dioxide|lead(IV) oxide]] as the anode) or by chlorine gas:<ref name="Greenwood872">Greenwood and Earnshaw, pp. 872–5</ref>

{{block indent|{{chem|IO|3|-}} + 6 OH<sup>−</sup> → {{chem|IO|6|5-}} + 3 H<sub>2</sub>O + 2 e<sup>−</sup>}}
{{block indent|{{chem|IO|3|-}} + 6 OH<sup>−</sup> + Cl<sub>2</sub> → {{chem|IO|6|5-}} + 2 Cl<sup>−</sup> + 3 H<sub>2</sub>O}}

They are thermodymically and kinetically powerful oxidising agents, quickly oxidising Mn<sup>2+</sup> to [[permanganate|{{chem|MnO|4|-}}]], and cleaving [[Diol|glycols]], α-[[Dicarbonyl|diketones]], α-[[Hydroxy ketone|ketols]], α-[[Alkanolamine|aminoalcohols]], and α-[[diamine]]s.<ref name="Greenwood872" /> Orthoperiodate especially stabilises high oxidation states among metals because of its very high negative charge of −5. [[Periodic acid|Orthoperiodic acid]], H<sub>5</sub>IO<sub>6</sub>, is stable, and dehydrates at 100&nbsp;°C in a vacuum to [[Periodic acid|Metaperiodic acid]], HIO<sub>4</sub>. Attempting to go further does not result in the nonexistent iodine heptoxide (I<sub>2</sub>O<sub>7</sub>), but rather iodine pentoxide and oxygen. Periodic acid may be protonated by [[sulfuric acid]] to give the {{chem|I(OH)|6|+}} cation, isoelectronic to Te(OH)<sub>6</sub> and {{chem|Sb(OH)|6|-}}, and giving salts with bisulfate and sulfate.<ref name="King" />

===Polyiodine compounds===
When iodine dissolves in strong acids, such as fuming sulfuric acid, a bright blue [[Paramagnetism|paramagnetic]] solution including {{chem|I|2|+}} cations is formed. A solid salt of the diiodine cation may be obtained by oxidising iodine with [[antimony pentafluoride]]:<ref name="King" />

{{block indent|2 I<sub>2</sub> + 5 SbF<sub>5</sub> {{overunderset|{{big|⟶}}|SO<sub>2</sub>|20&nbsp;°C}} 2 I<sub>2</sub>Sb<sub>2</sub>F<sub>11</sub> + SbF<sub>3</sub>}}

The salt I<sub>2</sub>Sb<sub>2</sub>F<sub>11</sub> is dark blue, and the blue [[tantalum]] analogue I<sub>2</sub>Ta<sub>2</sub>F<sub>11</sub> is also known. Whereas the I–I bond length in I<sub>2</sub> is 267&nbsp;pm, that in {{chem|I|2|+}} is only 256&nbsp;pm as the missing electron in the latter has been removed from an antibonding orbital, making the bond stronger and hence shorter. In [[fluorosulfuric acid]] solution, deep-blue {{chem|I|2|+}} reversibly dimerises below −60&nbsp;°C, forming red rectangular diamagnetic {{chem|I|4|2+}}. Other polyiodine cations are not as well-characterised, including bent dark-brown or black {{chem|I|3|+}} and centrosymmetric ''C''<sub>2''h''</sub> green or black {{chem|I|5|+}}, known in the {{chem|AsF|6|-}} and {{chem|AlCl|4|-}} salts among others.<ref name="King" /><ref name="Greenwood842">Greenwood and Earnshaw, pp. 842–4</ref>

The only important polyiodide anion in aqueous solution is linear [[triiodide]], {{chem|I|3|-}}. Its formation explains why the solubility of iodine in water may be increased by the addition of potassium iodide solution:<ref name="King" />

{{block indent|I<sub>2</sub> + I<sup>−</sup> {{eqm}} {{chem|I|3|-}} (''K''<sub>eq</sub> {{=}} c. 700 at 20&nbsp;°C)}}

Many other polyiodides may be found when solutions containing iodine and iodide crystallise, such as {{chem|I|5|-}}, {{chem|I|9|-}}, {{chem|I|4|2-}}, and {{chem|I|8|2-}}, whose salts with large, weakly polarising cations such as [[caesium|Cs<sup>+</sup>]] may be isolated.<ref name="King" /><ref name="Greenwood835">Greenwood and Earnshaw, pp. 835–9</ref>

===Organoiodine compounds===
{{main|Organoiodine compound}}
[[File:IBXAcid.png|thumb|right|Structure of the oxidising agent [[2-Iodoxybenzoic acid|2-iodoxybenzoic acid]]]]
Organoiodine compounds have been fundamental in the development of organic synthesis, such as in the [[Hofmann elimination]] of [[amine]]s,<ref>{{cite journal | title = Beiträge zur Kenntniss der flüchtigen organischen Basen | journal = [[Annalen der Chemie und Pharmacie]] | volume = 78 | issue = 3 | year = 1851 | pages = 253–286 | vauthors = Hofmann AW | doi = 10.1002/jlac.18510780302 | url = https://zenodo.org/record/1427040 | access-date = 30 June 2019 | archive-date = 1 December 2022 | archive-url = https://web.archive.org/web/20221201072415/https://zenodo.org/record/1427040 | url-status = live }}</ref> the [[Williamson ether synthesis]],<ref>{{cite journal | title = Theory of Aetherification | journal = Philosophical Magazine | volume = 37 | issue = 251 | pages = 350–356 | year = 1850 | doi = 10.1080/14786445008646627 | vauthors = Williamson A | url = https://zenodo.org/record/1431121 | access-date = 29 September 2020 | archive-date = 9 November 2022 | archive-url = https://web.archive.org/web/20221109194527/https://zenodo.org/record/1431121 | url-status = live }} ([http://web.lemoyne.edu/~giunta/williamson.html Link to excerpt]. {{Webarchive|url=https://web.archive.org/web/20190423075534/http://web.lemoyne.edu/~giunta/williamson.html |date=23 April 2019 }})</ref> the [[Wurtz reaction|Wurtz coupling reaction]],<ref>{{cite journal | title = Ueber eine neue Klasse organischer Radicale | vauthors = Wurtz A | journal = [[Annalen der Chemie und Pharmacie]] | volume = 96 | issue = 3 | pages = 364–375 | year = 1855 | url = https://zenodo.org/record/1427074 | doi = 10.1002/jlac.18550960310 | access-date = 30 June 2019 | archive-date = 3 February 2023 | archive-url = https://web.archive.org/web/20230203205851/https://zenodo.org/record/1427074 | url-status = live }}</ref> and in [[Grignard reagent]]s.<ref>{{cite journal | vauthors = Grignard V | title = Sur quelques nouvelles combinaisons organométaliques du magnésium et leur application à des synthèses d'alcools et d'hydrocabures | journal = Comptes rendus de l'Académie des Sciences | year = 1900 | volume = 130 | pages = 1322–25 | url = http://gallica.bnf.fr/ark:/12148/bpt6k3086n/f1322.table | author-link = Victor Grignard | access-date = 2 October 2016 | archive-date = 8 August 2019 | archive-url = https://web.archive.org/web/20190808225609/https://gallica.bnf.fr/ark:/12148/bpt6k3086n/f1322.table | url-status = live }}</ref>

The [[carbon]]–iodine bond is a common functional group that forms part of core [[organic chemistry]]; formally, these compounds may be thought of as organic derivatives of the [[Iodide|iodide anion]]. The simplest [[Organoiodine chemistry|organoiodine compounds]], [[Organoiodine chemistry|alkyl iodides]], may be synthesised by the reaction of [[Alcohol (chemistry)|alcohol]]s with [[phosphorus triiodide]]; these may then be used in [[nucleophilic substitution]] reactions, or for preparing [[Grignard reagent]]s. The C–I bond is the weakest of all the carbon–halogen bonds due to the minuscule difference in electronegativity between carbon (2.55) and iodine (2.66). As such, iodide is the best [[leaving group]] among the halogens, to such an extent that many organoiodine compounds turn yellow when stored over time due to decomposition into elemental iodine; as such, they are commonly used in [[organic synthesis]], because of the easy formation and cleavage of the C–I bond.<ref>{{Ullmann | vauthors = Lyday PA | title = Iodine and Iodine Compounds | doi = 10.1002/14356007.a14_381}}</ref> They are also significantly denser than the other organohalogen compounds thanks to the high atomic weight of iodine.<ref name="blanksby">{{cite journal | vauthors = Blanksby SJ, Ellison GB | title = Bond dissociation energies of organic molecules | journal = Accounts of Chemical Research | volume = 36 | issue = 4 | pages = 255–263 | date = April 2003 | pmid = 12693923 | doi = 10.1021/ar020230d | url = http://www.colorado.edu/chem/ellison/papers/Blanksby_Acct_Chem_Res_2003.pdf | access-date = 25 October 2017 | citeseerx = 10.1.1.616.3043 | archive-url = https://web.archive.org/web/20090206144739/http://colorado.edu/chem/ellison/papers/Blanksby_Acct_Chem_Res_2003.pdf | archive-date = 6 February 2009 }}</ref> A few organic oxidising agents like the [[Hypervalent organoiodine compounds|iodanes]] contain iodine in a higher oxidation state than −1, such as [[2-Iodoxybenzoic acid|2-iodoxybenzoic acid]], a common reagent for the oxidation of alcohols to [[aldehyde]]s,<ref>{{ OrgSynth | title = Dess–Martin periodinane: 1,1,1-Triacetoxy-1,1-dihydro-1,2-benziodoxol-3(1''H'')-one | vauthors = Boeckman Jr RK, Shao P, Mullins JJ | year = 2000 | volume = 77 | pages = 141 | collvol = 10 | collvolpages = 696 | prep = v77p0141 }}</ref> and [[iodobenzene dichloride]] (PhICl<sub>2</sub>), used for the selective chlorination of [[alkene]]s and [[alkyne]]s.<ref>{{cite journal | vauthors = Jung ME, Parker MH | title = Synthesis of Several Naturally Occurring Polyhalogenated Monoterpenes of the Halomon Class(1) | journal = The Journal of Organic Chemistry | volume = 62 | issue = 21 | pages = 7094–7095 | date = October 1997 | pmid = 11671809 | doi = 10.1021/jo971371 }}</ref> One of the more well-known uses of organoiodine compounds is the so-called [[Haloform reaction|iodoform test]], where [[iodoform]] (CHI<sub>3</sub>) is produced by the exhaustive iodination of a [[Ketone|methyl ketone]] (or another compound capable of being oxidised to a methyl ketone), as follows:<ref name="March">{{March6th}}</ref>

{{block indent|[[File:Iodoform synthesis.svg|450px|class=skin-invert-image]]}}

Some drawbacks of using organoiodine compounds as compared to organochlorine or organobromine compounds is the greater expense and toxicity of the iodine derivatives, since iodine is expensive and organoiodine compounds are stronger alkylating agents.<ref>{{cite web|publisher = Oxford University|title = Safety data for iodomethane|url = http://msds.chem.ox.ac.uk/IO/iodomethane.html|access-date = 12 December 2008|archive-date = 10 August 2010|archive-url = https://web.archive.org/web/20100810211004/http://msds.chem.ox.ac.uk/IO/iodomethane.html}}</ref> For example, [[iodoacetamide]] and [[iodoacetic acid]] denature proteins by irreversibly alkylating [[cysteine]] residues and preventing the reformation of [[disulfide]] linkages.<ref>{{cite journal | vauthors = Polgár L | title = Deuterium isotope effects on papain acylation. Evidence for lack of general base catalysis and for enzyme–leaving-group interaction | journal = European Journal of Biochemistry | volume = 98 | issue = 2 | pages = 369–374 | date = August 1979 | pmid = 488108 | doi = 10.1111/j.1432-1033.1979.tb13196.x | doi-access = free }}</ref>

Halogen exchange to produce iodoalkanes by the [[Finkelstein reaction]] is slightly complicated by the fact that iodide is a better leaving group than chloride or bromide. The difference is nevertheless small enough that the reaction can be driven to completion by exploiting the differential solubility of halide salts, or by using a large excess of the halide salt.<ref name="March" /> In the classic Finkelstein reaction, an [[Organochlorine chemistry|alkyl chloride]] or an [[Organobromine chemistry|alkyl bromide]] is converted to an [[Organoiodine chemistry|alkyl iodide]] by treatment with a solution of [[sodium iodide]] in [[acetone]]. Sodium iodide is soluble in acetone and [[sodium chloride]] and [[sodium bromide]] are not.<ref>{{cite journal | vauthors = Ervithayasuporn V, Ervithayasuporn V, Pornsamutsin N, Pornsamutsin N, Prangyoo P, Prangyoo P, Sammawutthichai K, Sammawutthichai K, Jaroentomeechai T, Jaroentomeechai T, Phurat C, Phurat C, Teerawatananond T, Teerawatananond T | title = One-pot synthesis of halogen exchanged silsesquioxanes: octakis(3-bromopropyl)octasilsesquioxane and octakis(3-iodopropyl)octasilsesquioxane | journal = Dalton Transactions | volume = 42 | issue = 37 | pages = 13747–13753 | date = October 2013 | pmid = 23907310 | doi = 10.1039/C3DT51373D | s2cid = 41232118 }}</ref> The reaction is driven toward products by [[Law of mass action|mass action]] due to the precipitation of the insoluble salt.<ref>{{cite journal | vauthors = Streitwieser A | year = 1956 | title = Solvolytic Displacement Reactions at Saturated Carbon Atoms | journal = [[Chemical Reviews]] | volume = 56 | pages = 571–752 | doi = 10.1021/cr50010a001 | issue = 4}}</ref><ref>{{cite journal | title = The Effect of the Carbonyl and Related Groups on the Reactivity of Halides in S<sub>N</sub>2 Reactions | vauthors = Bordwell FG, Brannen WT | journal = [[Journal of the American Chemical Society]] | year = 1964 | volume = 86 | pages = 4645–4650 | doi = 10.1021/ja01075a025 | issue = 21| bibcode = 1964JAChS..86.4645B }}</ref>

==Occurrence and production==
Iodine is the least abundant of the stable halogens, comprising only 0.46&nbsp;[[Parts-per notation|parts per million]] of Earth's crustal rocks (compare: [[fluorine]]: 544&nbsp;ppm, [[chlorine]]: 126&nbsp;ppm, [[bromine]]: 2.5&nbsp;ppm) making it the 60th most abundant element.<ref name="Greenwood795">Greenwood and Earnshaw, pp. 795–796.</ref> Iodide minerals are rare, and most deposits that are concentrated enough for economical extraction are iodate minerals instead. Examples include [[Calcium iodate|lautarite]], Ca(IO<sub>3</sub>)<sub>2</sub>, and dietzeite, 7Ca(IO<sub>3</sub>)<sub>2</sub>·8CaCrO<sub>4</sub>.<ref name="Greenwood795" /> These are the minerals that occur as trace impurities in the [[caliche]], found in [[Chile]], whose main product is [[sodium nitrate]]. In total, they can contain at least 0.02% and at most 1% iodine by mass.<ref name="Elzea">{{cite book |title = Industrial Minerals & Rocks: Commodities, Markets, and Uses |publisher = SME |date = 2006 |isbn = 978-0-87335-233-8 |url = https://books.google.com/books?id=zNicdkuulE4C |pages = 541–552 | veditors = Kogel JE, Trivedi NC, Barker JM, Krukowski ST }}</ref> [[Sodium iodate]] is extracted from the caliche and reduced to iodide by [[sodium bisulfite]]. This solution is then reacted with freshly extracted iodate, resulting in [[comproportionation]] to iodine, which may be filtered off.<ref name="Greenwood800" />

The caliche was the main source of iodine in the 19th century and continues to be important today, replacing [[kelp]] (which is no longer an economically viable source),<ref>{{cite journal |url = https://books.google.com/books?id=wW8KAAAAIAAJ&pg=PA185 | vauthors = Stanford EC |journal = Journal of the Society of Arts |title = On the Economic Applications of Seaweed |date = 1862 |pages = 185–189}}</ref> but in the late 20th century [[brine]]s emerged as a comparable source. The Japanese [[Minami Kantō gas field]] east of [[Tokyo]] and the American [[Anadarko Basin]] gas field in northwest [[Oklahoma]] are the two largest such sources. The brine is hotter than 60&nbsp;°C from the depth of the source. The brine is first [[List of purification methods in chemistry|purified]] and acidified using [[sulfuric acid]], then the iodide present is oxidised to iodine with chlorine. An iodine solution is produced, but is dilute and must be concentrated. Air is blown into the solution to [[Evaporation|evaporate]] the iodine, which is passed into an absorbing tower, where [[sulfur dioxide]] reduces the iodine. The [[hydrogen iodide]] (HI) is reacted with chlorine to precipitate the iodine. After filtering and purification the iodine is packed.<ref name="Elzea" /><ref>{{cite journal |journal = Geochemical Journal |volume = 40 |page = 475 |date = 2006 |title = Chemical and isotopic compositions of brines from dissolved-in-water type natural gas fields in Chiba, Japan | vauthors = Maekawa T, Igari SI, Kaneko N |doi = 10.2343/geochemj.40.475 |issue = 5 |bibcode = 2006GeocJ..40..475M|doi-access = free }}</ref>

{{block indent| 2 HI + Cl<sub>2</sub> → I<sub>2</sub>↑ + 2 HCl}}
{{block indent| I<sub>2</sub> + 2 H<sub>2</sub>O + SO<sub>2</sub> → 2 HI + H<sub>2</sub>SO<sub>4</sub>}}
{{block indent| 2 HI + Cl<sub>2</sub> → I<sub>2</sub>↓ + 2 HCl}}

These sources ensure that Chile and Japan are the largest producers of iodine today.<ref name="Greenwood795" /> The companies [[Sociedad Química y Minera]] (SQM) and [[Cosayach]] lead iodine mining in Chile.<ref>{{Cite news |title=Algorta Norte busca estar entre los tres principales produtores de yodo |date=2012-03-19 |url=https://www.df.cl/empresas/algorta-norte-busca-estar-entre-los-tres-mayores-productores-de-yodo |access-date=2025-08-07 |work=[[Diario Financiero]] |language=es}}</ref> Alternatively, the brine may be treated with [[silver nitrate]] to precipitate out iodine as [[silver iodide]], which is then decomposed by reaction with iron to form metallic silver and a solution of [[iron(II) iodide]]. The iodine is then liberated by displacement with [[chlorine]].<ref name="Greenwood799">Greenwood and Earnshaw, p. 799.</ref>

Large purple clouds of sublimated iodine have been observed at iodine extraction sites.<ref>{{Cite news |title=Autoridades monitorean evento que dejó una nube púrpura de vapor de yodo en Pozo Almonte |date=2022-08-21 |url=https://cooperativa.cl/noticias/pais/region-de-tarapaca/autoridades-monitorean-evento-que-dejo-una-nube-purpura-de-vapor-de-yodo/2022-08-21/204433.html |access-date=2025-08-04 |work=[[Radio Bío-Bío]] |language=es}}</ref>

==Applications==
About half of all produced iodine goes into various [[Organoiodine chemistry|organoiodine compounds]], another 15% remains as the pure element, another 15% is used to form [[potassium iodide]], and another 15% for other [[Iodine compounds|inorganic iodine compounds]].<ref name="Greenwood800" /> Among the major uses of iodine compounds are [[Catalysis|catalysts]], animal feed supplements, stabilisers, dyes, colourants and pigments, pharmaceutical, sanitation (from [[tincture of iodine]]), and photography; minor uses include [[Smog tower|smog inhibition]], [[cloud seeding]], and various uses in [[analytical chemistry]].<ref name="Greenwood800" />

===X-ray imaging===
As an element with high [[electron density]] and atomic number, iodine efficiently absorbs X-rays.  X-ray [[Radiocontrast agent|radiocontrast]] agents is the top application for iodine.<ref name="Ullmann"/>  In this application, Organoiodine compounds are injected intravenously. This application is often in conjunction with advanced X-ray techniques such as [[angiography]] and [[CT scan]]ning. At present, all water-soluble radiocontrast agents rely on [[Iodinated contrast|iodine-containing compounds]].

Iodine absorbs X-rays with energies less than 33.3&nbsp;keV due to the [[photoelectric effect]] of the innermost electrons.<ref>{{cite book | vauthors = Lancaster JL | chapter-url = http://ric.uthscsa.edu/personalpages/lancaster/DI-II_Chapters/DI_chap4.pdf | chapter = Chapter 4: Physical Determinants of Contrast | archive-url =https://web.archive.org/web/20151010172937/http://ric.uthscsa.edu/personalpages/lancaster/DI-II_Chapters/DI_chap4.pdf | archive-date=10 October 2015 | title = Physics of Medical X-Ray Imaging | publisher = The University of Texas Health Science Center }}</ref> 

===Biocide===
{{Main|Iodine (medical use)}}
[[File:Diatrizoic acid.svg|thumb|right|[[Diatrizoate|Diatrizoic acid]], an iodine-containing [[radiocontrast agent]]]]

Use of iodine as a biocide represents a major application of the element, ranked 2nd by weight.<ref name="Ullmann"/> Elemental iodine (I<sup>2</sup>) is used as an [[antiseptic]] in medicine.<ref name="WHO2008">{{cite book | title = WHO Model Formulary 2008 | year = 2009 | isbn = 978-92-4-154765-9 | vauthors = ((World Health Organization)) | veditors = Stuart MC, Kouimtzi M, Hill SR | hdl = 10665/44053 | author-link = World Health Organization | publisher = World Health Organization | hdl-access=free | page=499 }}</ref> A number of water-soluble compounds, from [[triiodide]] (I<sub>3</sub><sup>−</sup>, generated ''in situ'' by adding [[iodide]] to poorly water-soluble elemental iodine) to various [[iodophor]]s, slowly decompose to release I<sup>2</sup> when applied.<ref>{{cite book | vauthors = Block SS |title=Disinfection, sterilization, and preservation |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |date=2001 |page=159 |isbn=978-0-683-30740-5}}</ref>

===Optical polarising films===
[[TFT LCD|Thin-film-transistor liquid crystal displays]] rely on [[polarization (waves)| polarisation]]. The liquid crystal transistor is sandwiched between two polarising films and illuminated from behind. The two films prevent light transmission unless the transistor in the middle of the sandwich rotates the light.<ref>{{Cite journal |last1=Ma |first1=Ji |last2=Ye |first2=Xin |last3=Jin |first3=Bo |date=1 April 2011 |title=Structure and application of polarizer film for thin-film-transistor liquid crystal displays |url=https://linkinghub.elsevier.com/retrieve/pii/S0141938211000126 |journal=Displays |volume=32 |issue=2 |pages=49–57 |doi=10.1016/j.displa.2010.12.006 |issn=0141-9382|url-access=subscription }}</ref> Iodine-impregnated polymer films are used in  [[polarization (waves)| polarising]] optical components with the highest transmission and degree of polarisation.<ref>{{cite book |doi=10.1002/9781118909911.ch26 |chapter=Polarizing Films |title=Iodine Chemistry and Applications |date=2014 |last1=Kahr |first1=Bart |last2=Knowles |first2=Kevin M. |pages=479–488 |isbn=978-1-118-46629-2 }}</ref>

===Co-catalyst===
Another significant use of iodine is as a cocatalyst for the production of [[acetic acid]] by the [[Monsanto process|Monsanto]] and [[Cativa process]]es. In these technologies, [[hydroiodic acid]] converts the [[methanol]] feedstock into methyl iodide, which undergoes [[carbonylation]]. Hydrolysis of the resulting acetyl iodide regenerates hydroiodic acid and gives acetic acid. The majority of acetic acid is produced by these approaches.<ref name=UllmannAA>{{Ullmann | vauthors = Le Berre C, Serp P, Kalck, P, Torrence GP | title = Acetic Acid | doi = 10.1002/14356007.a01_045.pub3|year=2013|publisher=Wiley-VCH|location=Weinheim}}</ref><ref>{{Cite journal |last1=Sunley |first1=Glenn J |last2=Watson |first2=Derrick J |date=26 May 2000 |title=High productivity methanol carbonylation catalysis using iridium: The Cativa™ process for the manufacture of acetic acid |url=https://linkinghub.elsevier.com/retrieve/pii/S0920586100002637 |journal=Catalysis Today |volume=58 |issue=4 |pages=293–307 |doi=10.1016/S0920-5861(00)00263-7 |issn=0920-5861|url-access=subscription }}</ref>

===Nutrition===
Salts of iodide and iodate are used extensively in human and animal nutrition. This application reflects the status of iodide as an [[Mineral (nutrient)|essential element]], being required for two hormones.  The production of [[ethylenediamine dihydroiodide]], provided as a [[nutrition|nutritional supplement]] for livestock, consumes a large portion of available iodine.<ref name="Ullmann">{{cite book | vauthors = Lyday PA, Kaiho T | chapter = Iodine and Iodine Compounds | title = Ullmann's Encyclopedia of Industrial Chemistry | date = 2015 | publisher = Wiley-VCH | location = Weinheim | doi = 10.1002/14356007.a14_381.pub2 | volume = A14 | pages = 382–390 | isbn = 978-3-527-30673-2 }}</ref> Iodine is a component of [[iodised salt]]. 

A saturated solution of [[potassium iodide]] is used to treat acute [[Hyperthyroidism|thyrotoxicosis]]. It is also used to block uptake of [[iodine-131]] in the thyroid gland (see isotopes section above), when this isotope is used as part of radiopharmaceuticals (such as [[iobenguane]]) that are not targeted to the thyroid or thyroid-type tissues.<ref>{{cite web |url=http://hazard.com/msds/mf/baker/baker/files/p5906.htm |title=Solubility of KI in water |publisher=Hazard.com |date=21 April 1998 |access-date=21 January 2013 |archive-date=23 April 2012 |archive-url=https://web.archive.org/web/20120423195709/http://hazard.com/msds/mf/baker/baker/files/p5906.htm |url-status=live }}</ref><ref>{{cite web|url=http://www.eanm.org/scientific_info/guidelines/gl_radio_ther_benzyl.pdf|archive-url=https://web.archive.org/web/20090617073253/http://www.eanm.org/scientific_info/guidelines/gl_radio_ther_benzyl.pdf | title=EANM procedure guidelines for 131I-meta-iodobenzylguanidine (131I-mIBG) therapy|archive-date=17 June 2009|date=17 June 2009}}</ref>

===Others===
Inorganic iodides find specialised uses. [[Titanium]], [[zirconium]], [[hafnium]], and [[thorium]] are purified by the [[Van Arkel–de Boer process]], which involves the reversible formation of the tetraiodides of these elements. Silver iodide is a major ingredient to traditional photographic film. Thousands of kilograms of silver iodide are used annually for [[cloud seeding]] to induce rain.<ref name = Ullmann/>

The organoiodine compound [[erythrosine]] is an important food colouring agent. Perfluoroalkyl iodides are precursors to important surfactants, such as [[perfluorooctanesulfonic acid]].<ref name = Ullmann/>

{{sup|125}}I is used as the [[Radioactive tracer|radiolabel]] in investigating which [[ligand (biochemistry)|ligand]]s go to which [[Pattern recognition receptor|plant pattern recognition receptors]] (PRRs).<ref name="Boutrot-Zipfel-2017">{{cite journal | vauthors = Boutrot F, Zipfel C | title = Function, Discovery, and Exploitation of Plant Pattern Recognition Receptors for Broad-Spectrum Disease Resistance | journal = Annual Review of Phytopathology | volume = 55 | issue = 1 | pages = 257–286 | date = August 2017 | pmid = 28617654 | doi = 10.1146/annurev-phyto-080614-120106 | publisher = [[Annual Reviews (publisher)|Annual Reviews]] | doi-access = free | bibcode = 2017AnRvP..55..257B }}</ref>

An iodine based thermochemical cycle has been evaluated for hydrogen production using energy from nuclear power.<ref name="Corgnale-Review-2020"/> The cycle has  three steps. At {{convert|120|°C}}, iodine reacts with [[sulfur dioxide]] and water to give hydrogen iodide and [[sulfuric acid]]:
:{{chem2 | I2 + SO2 + 2 H2O ->  2 HI + H2SO4 }}
After a separation stage, at {{convert|830|–|850|°C}} sulfuric acid splits in sulfur dioxide and oxygen:
:{{chem2 | 2 H2SO4 ->  SO2 + 2 H2O + O2 }}
Hydrogen iodide, at {{convert|300|–|320|°C}}, gives hydrogen and the initial element, iodine:
:{{chem2 | 2 HI -> I2 + H2 }}
The yield of the cycle (ratio between lower heating value of the produced hydrogen and the consumed energy for its production, is approximately 38%. {{as of|2020}}, the cycle is not a competitive means of producing hydrogen.<ref name="Corgnale-Review-2020">{{Cite journal |last1=Corgnale |first1=Claudio |last2=Gorensek |first2=Maximilian B. |last3=Summers |first3=William A. |date=November 2020 |title=Review of Sulfuric Acid Decomposition Processes for Sulfur-Based Thermochemical Hydrogen Production Cycles |journal=Processes |language=en |volume=8 |issue=11 |page=1383 |doi=10.3390/pr8111383 |doi-access=free |issn=2227-9717}}</ref>

== Spectroscopy ==
The spectrum of the iodine molecule, I<sub>2</sub>, consists of (not exclusively) tens of thousands of sharp spectral lines in the wavelength range 500–700&nbsp;nm. It is therefore a commonly used wavelength reference (secondary standard). By measuring with a [[Saturated absorption spectroscopy|spectroscopic Doppler-free technique]] while focusing on one of these lines, the [[Hyperfine structure|hyperfine]] structure of the iodine molecule reveals itself. A line is now resolved such that either 15 components (from even rotational quantum numbers, ''J''<sub>even</sub>), or 21 components (from odd rotational quantum numbers, ''J''<sub>odd</sub>) are measurable.<ref>{{cite journal | vauthors = Sansonetti CJ | title = Precise measurements of hyperfine components in the spectrum of molecular iodine | journal = Journal of the Optical Society of America B | date = August 1997 | volume = 14 | issue = 8 | pages = 1913–1920 | language = en | osti = 464573 | doi = 10.2172/464573 | url = https://digital.library.unt.edu/ark:/67531/metadc684462/ | access-date = 11 January 2020 | archive-date = 4 June 2021 | archive-url = https://web.archive.org/web/20210604044642/https://digital.library.unt.edu/ark:/67531/metadc684462/ | url-status = live }}</ref>

Caesium iodide and thallium-doped sodium iodide are used in crystal [[scintillator]]s for the detection of gamma rays. The efficiency is high and energy dispersive spectroscopy is possible, but the resolution is rather poor.

==Chemical analysis==
[[File:Testing seed for starch.jpg|thumb|Testing a seed for starch with a solution of iodine]]
The iodide and iodate anions can be used for quantitative volumetric analysis, for example in [[iodometry]]. Iodine and starch form a blue complex, and this reaction is often used to test for either starch or iodine and as an [[Redox indicator|indicator]] in iodometry. The iodine test for starch is still used to detect [[counterfeit]] banknotes printed on starch-containing paper.<ref name="Emsley">{{cite book | vauthors = Emsley J | title = Nature's Building Blocks | edition = Hardcover, First | publisher = [[Oxford University Press]] | date = 2001 | pages = [https://archive.org/details/naturesbuildingb0000emsl/page/244 244–250] | isbn = 978-0-19-850340-8 | url = https://archive.org/details/naturesbuildingb0000emsl/page/244 }}</ref>

The [[iodine value]] is the mass of iodine in grams that is consumed by 100 grams of a [[chemical substance]] typically fats or oils. Iodine numbers are often used to determine the amount of unsaturation in [[fatty acid]]s. This unsaturation is in the form of [[double bond]]s, which react with iodine compounds.

[[Potassium tetraiodomercurate(II)]], K<sub>2</sub>HgI<sub>4</sub>, is also known as Nessler's reagent. It is once was used as a sensitive spot test for [[ammonia]]. Similarly, [[Mayer's reagent]] (potassium tetraiodomercurate(II) solution) is used as a precipitating reagent to test for [[alkaloid]]s.<ref>{{cite journal | last1=Szász | first1=György | last2=Buda | first2=László | title=Contribution to the reaction of alkaloids with potassium tetraiodomercurate | journal=Fresenius' Zeitschrift für Analytische Chemie| publisher=Springer Science and Business Media LLC | volume=253 | issue=5 | year=1971 | issn=0016-1152 | doi=10.1007/bf00426350 | pages=361–363| s2cid=91439011 }}</ref> Aqueous alkaline iodine solution is used in the [[iodoform]] test for [[Ketone|methyl ketones]].<ref name="March" />

==Biological role==
{{main|Iodine in biology}}
[[File:Thyroid system.svg|thumb|upright=1.2|The [[thyroid]] system of the thyroid hormones [[triiodothyronine|T<sub>3</sub>]] and [[Levothyroxine|T<sub>4</sub>]]]]
[[File:Carte iodurie france µg par jour d'après Mornex 1987 Le Guen 2000.jpg|thumb|upright=1.2|Comparison of the iodine content in urine in [[France]] (in microgramme/day), for some regions and departments (average levels of urine iodine, measured in micrograms per litre at the end of the twentieth century (1980 to 2000))<ref>Mornex, 1987 and Le Guen et al., 2000, cited by {{cite journal | vauthors = Le Guen B, Hemidy PY, Gonin M, Bailloeuil C, Van Boxsom D, Renier S, Garcier Y | year = 2001 | title = Arguments et retour d'expérience sur la distribution d'iode stable autour des centrales nucléaires françaises | url = https://www.researchgate.net/publication/245276139 | journal = Radioprotection | volume = 36 | issue = 4| pages = 417–430 | doi = 10.1051/radiopro:2001101 | doi-access = free }}</ref>]]
Iodine is an [[Mineral (nutrient)|essential element]] for life and, at atomic number ''Z'' = 53, is the heaviest element commonly needed by living organisms. ([[Lanthanum]] and the other [[lanthanide]]s, as well as [[tungsten]] with ''Z'' = 74 and [[uranium]] with ''Z'' = 92, are used by a few microorganisms.<ref>{{cite journal | vauthors = Pol A, Barends TR, Dietl A, Khadem AF, Eygensteyn J, Jetten MS, Op den Camp HJ | title = Rare earth metals are essential for methanotrophic life in volcanic mudpots | journal = Environmental Microbiology | volume = 16 | issue = 1 | pages = 255–264 | date = January 2014 | pmid = 24034209 | doi = 10.1111/1462-2920.12249 | bibcode = 2014EnvMi..16..255P | url = https://repository.ubn.ru.nl//bitstream/handle/2066/128108/128108.pdf | access-date = 17 January 2024 | archive-date = 17 January 2024 | archive-url = https://web.archive.org/web/20240117120818/https://repository.ubn.ru.nl//bitstream/handle/2066/128108/128108.pdf | url-status = live }}</ref><ref>{{cite journal| title = Identification of molybdopterin as the organic component of the tungsten cofactor in four enzymes from hyperthermophilic Archaea | author = Johnson JL, Rajagopalan KV, Mukund S, Adams MW. | journal = [[Journal of Biological Chemistry]] |date = 5 March 1993 |volume =268 |issue=7 | pages = 4848–52| doi = 10.1016/S0021-9258(18)53474-8 |pmid= 8444863 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Koribanics NM, Tuorto SJ, Lopez-Chiaffarelli N, McGuinness LR, Häggblom MM, Williams KH, Long PE, Kerkhof LJ | title = Spatial distribution of an uranium-respiring betaproteobacterium at the Rifle, CO field research site | journal = PLOS ONE | volume = 10 | issue = 4 | article-number = e0123378 | year = 2015 | pmid = 25874721 | pmc = 4395306 | doi = 10.1371/journal.pone.0123378 | doi-access = free | bibcode = 2015PLoSO..1023378K }}</ref>) It is required for the synthesis of the growth-regulating thyroid hormones [[Levothyroxine|tetraiodothyronine]] and [[triiodothyronine]] (T<sub>4</sub> and T<sub>3</sub> respectively, named after their number of iodine atoms). A deficiency of iodine leads to decreased production of T<sub>3</sub> and T<sub>4</sub> and a concomitant enlargement of the [[thyroid|thyroid tissue]] in an attempt to obtain more iodine, causing the disease [[goitre]]. The major form of thyroid hormone in the blood is tetraiodothyronine (T<sub>4</sub>), which has a longer life than triiodothyronine (T<sub>3</sub>). In humans, the ratio of T<sub>4</sub> to T<sub>3</sub> released into the blood is between 14:1 and 20:1. T<sub>4</sub> is converted to the active T<sub>3</sub> (three to four times more potent than T<sub>4</sub>) within [[cell (biology)|cells]] by [[deiodinase]]s (5'-iodinase). These are further processed by [[decarboxylation]] and deiodination to produce [[3-Iodothyronamine|iodothyronamine]] (T<sub>1</sub>a) and [[thyronamine]] (T<sub>0</sub>a'). All three isoforms of the deiodinases are [[selenium]]-containing enzymes; thus metallic selenium is needed for triiodothyronine and tetraiodothyronine production.<ref>{{cite web|url=http://emedicine.medscape.com/article/819692-overview#showall|vauthors=Irizarry L|title=Thyroid Hormone Toxicity|website=Medscape|publisher=WedMD LLC|date=23 April 2014|access-date=2 May 2014|archive-date=31 October 2021|archive-url=https://web.archive.org/web/20211031132146/https://emedicine.medscape.com/article/819692-overview#showall|url-status=live}}</ref>

Iodine accounts for 65% of the molecular weight of T<sub>4</sub> and 59% of T<sub>3</sub>. Fifteen to 20&nbsp;mg of iodine is concentrated in thyroid tissue and hormones, but 70% of all iodine in the body is found in other tissues, including mammary glands, [[Eye|eyes]], gastric mucosa, thymus, [[cerebrospinal fluid]], choroid plexus, arteries, [[cervix]], salivary glands. During pregnancy, the [[placenta]] is able to store and accumulate iodine.<ref>{{cite journal |last1=Burns |first1=R |last2=O'Herlihy |first2=C |last3=Smyth |first3=PP |title=The placenta as a compensatory iodine storage organ. |journal=Thyroid |date=May 2011 |volume=21 |issue=5 |pages=541–6 |doi=10.1089/thy.2010.0203 |pmid=21417918}}</ref><ref>{{cite journal |last1=Neven |first1=KY |last2=Marien |first2=CBD |last3=Janssen |first3=BG |last4=Roels |first4=HA |last5=Waegeneers |first5=N |last6=Nawrot |first6=TS |last7=Ruttens |first7=A |title=Variability of iodine concentrations in the human placenta. |journal=Scientific Reports |date=13 January 2020 |volume=10 |issue=1 |page=161 |doi=10.1038/s41598-019-56775-3 |pmid=31932629|pmc=6957482 |bibcode=2020NatSR..10..161N }}</ref> In the cells of those tissues, iodine enters directly by [[Sodium/iodide cotransporter|sodium-iodide symporter]] (NIS). The action of iodine in mammal tissues is related to fetal and neonatal development, and in the other tissues, it is known.<ref name="Patrick2008" /> 

===Dietary recommendations and intake===
The daily levels of intake recommended by the [[United States]] [[National Academy of Medicine]] are between 110 and 130 [[microgram|μg]] for infants up to 12 months, 90&nbsp;μg for children up to eight years, 130&nbsp;μg for children up to 13 years, 150&nbsp;μg for adults, 220&nbsp;μg for pregnant women and 290&nbsp;μg for lactating women.<ref name="lpi" /><ref>{{cite web|url=http://iom.edu/en/Global/News%20Announcements/~/media/Files/Activity%20Files/Nutrition/DRIs/DRISummaryListing2.ashx |archive-url=https://web.archive.org/web/20091030004039/http://iom.edu/en/Global/News%20Announcements/~/media/Files/Activity%20Files/Nutrition/DRIs/DRISummaryListing2.ashx |archive-date=30 October 2009 |title=Dietary Reference Intakes (DRIs): Recommended Intakes for Individuals, Vitamins |publisher=[[Institute of Medicine]] |date=2004 |access-date=9 June 2010 }}</ref> The Tolerable Upper Intake Level (TUIL) for adults is 1,100&nbsp;μg/day.<ref name="InstituteofMedicine">{{cite book| author = United States National Research Council| date = 2000| title = Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc| pages = 258–259| publisher = National Academies Press| url = http://books.nap.edu/openbook.php?record_id=10026&page=258| doi = 10.17226/10026| pmid = 25057538| bibcode = 2000nap..book10026I| isbn = 978-0-309-07279-3| access-date = 9 March 2008| archive-date = 25 July 2015| archive-url = https://web.archive.org/web/20150725203752/http://books.nap.edu/openbook.php?record_id=10026&page=258| url-status = live}}</ref> This upper limit was assessed by analysing the effect of supplementation on [[thyroid-stimulating hormone]].<ref name="Patrick2008" />

The [[European Food Safety Authority]] (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR; AI and UL are defined the same as in the United States. For women and men ages 18 and older, the PRI for iodine is set at 150&nbsp;μg/day; the PRI during pregnancy and lactation is 200&nbsp;μg/day. For children aged 1–17 years, the PRI increases with age from 90 to 130&nbsp;μg/day. These PRIs are comparable to the U.S. RDAs with the exception of that for lactation.<ref>{{cite web| title = Overview on Dietary Reference Values for the EU population as derived by the EFSA Panel on Dietetic Products, Nutrition and Allergies| year = 2017| url = https://www.efsa.europa.eu/sites/default/files/assets/DRV_Summary_tables_jan_17.pdf| access-date = 3 December 2023| archive-date = 28 August 2017| archive-url = https://web.archive.org/web/20170828082247/https://www.efsa.europa.eu/sites/default/files/assets/DRV_Summary_tables_jan_17.pdf| url-status = live}}</ref>

The thyroid gland needs 70&nbsp;μg/day of iodine to synthesise the requisite daily amounts of T4 and T3.<ref name="lpi" /> The higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of body systems, including [[Mammary gland|mammary glands]], [[gastric mucosa]], [[salivary gland]]s, [[Brain cell|brain cells]], [[choroid plexus]], [[thymus]], [[artery|arteries]].<ref name="lpi" /><ref>{{cite journal | vauthors = Venturi S, Venturi M | title = Iodine, thymus, and immunity | journal = Nutrition | volume = 25 | issue = 9 | pages = 977–979 | date = September 2009 | pmid = 19647627 | doi = 10.1016/j.nut.2009.06.002 }}</ref><ref>{{cite journal | vauthors = Ullberg S, Ewaldsson B | title = Distribution of radio-iodine studied by whole-body autoradiography | journal = Acta Radiologica | volume = 2 | pages = 24–32 | date = February 1964 | pmid = 14153759 | doi = 10.3109/02841866409134127 }}</ref><ref name="Venturi, Sebastiano 2014 185–205">{{cite journal| vauthors = Venturi S |title=Iodine, PUFAs and Iodolipids in Health and Disease: An Evolutionary Perspective|journal=Human Evolution|volume= 29 |issue= 1–3|pages=185–205|year=2014|issn=0393-9375}}</ref>

Natural food sources of iodine include [[seafood]] which contains [[fish]], [[Seaweed|seaweeds]], [[kelp]], [[shellfish]] and other [[Food|foods]] which contain [[Dairy product|dairy products]], [[Eggs as food|eggs]], [[Meat|meats]], [[Vegetable|vegetables]], so long as the animals ate iodine richly, and the plants are grown on iodine-rich soil.<ref>{{cite web| publisher =Iodine Global Network|url =http://ign.org/p142002146.html|archive-url=https://web.archive.org/web/20150813130042/http://ign.org/p142002146.html|archive-date=13 August 2015|title=Where do we get iodine from?|url-status=live}}</ref><ref name = medlineplus002421/> [[Iodised salt]] is fortified with [[potassium iodate]], a salt of iodine, potassium, oxygen.<ref name="medlineplus002421">{{cite encyclopedia| url = https://medlineplus.gov/ency/article/002421.htm| title = Iodine in diet| encyclopedia = MedlinePlus Medical Encyclopedia| access-date = 7 April 2016| archive-date = 5 July 2016| archive-url = https://web.archive.org/web/20160705122918/https://www.nlm.nih.gov/medlineplus/ency/article/002421.htm| url-status = live}}</ref><ref name="American Thyroid Association">{{cite web|title=American Thyroid Association|url=http://www.thyroid.org/iodine-deficiency/|work=thyroid.org|publisher=American Thyroid Association|access-date=4 April 2014|archive-date=3 August 2023|archive-url=https://web.archive.org/web/20230803045045/https://www.thyroid.org/iodine-deficiency/}}</ref>

As of 2000, the median intake of iodine from food in the United States was 240 to 300&nbsp;μg/day for men and 190 to 210&nbsp;μg/day for women.<ref name="InstituteofMedicine" /> The general US population has adequate iodine nutrition,<ref>{{cite journal | vauthors = Caldwell KL, Makhmudov A, Ely E, Jones RL, Wang RY | title = Iodine status of the U.S. population, National Health and Nutrition Examination Survey, 2005–2006 and 2007–2008 | journal = Thyroid | volume = 21 | issue = 4 | pages = 419–427 | date = April 2011 | pmid = 21323596 | doi = 10.1089/thy.2010.0077 | url = https://zenodo.org/record/1235283 | access-date = 29 September 2020 | archive-date = 2 December 2022 | archive-url = https://web.archive.org/web/20221202135223/https://zenodo.org/record/1235283 | url-status = live }}</ref><ref name="Lueng">{{cite journal | vauthors = Leung AM, Braverman LE, Pearce EN | title = History of U.S. iodine fortification and supplementation | journal = Nutrients | volume = 4 | issue = 11 | pages = 1740–1746 | date = November 2012 | pmid = 23201844 | pmc = 3509517 | doi = 10.3390/nu4111740 | doi-access = free }}</ref> with lactating women and pregnant women having a mild risk of deficiency.<ref name="Lueng" /> In Japan, consumption was considered much higher, ranging between 5,280&nbsp;μg/day to 13,800&nbsp;μg/day from [[wakame]] and [[kombu]] that are eaten,<ref name="Patrick2008">{{cite journal | vauthors = Patrick L | title = Iodine: deficiency and therapeutic considerations | journal = Alternative Medicine Review | volume = 13 | issue = 2 | pages = 116–127 | date = June 2008 | pmid = 18590348 | url = http://www.thorne.com/altmedrev/.fulltext/13/2/116.pdf | archive-url = https://web.archive.org/web/20130531112100/http://www.thorne.com/altmedrev/.fulltext/13/2/116.pdf | archive-date = 31 May 2013 }}</ref> both in the form of kombu and wakame and kombu and wakame [[umami]] [[Extract|extracts]] for [[Stock (food)|soup stock]] and [[Potato chip|potato chips]]. However, new studies suggest that Japan's consumption is closer to 1,000–3,000&nbsp;μg/day.<ref>{{cite journal | vauthors = Zava TT, Zava DT | title = Assessment of Japanese iodine intake based on seaweed consumption in Japan: A literature-based analysis | journal = Thyroid Research | volume = 4 | page = 14 | date = October 2011 | pmid = 21975053 | pmc = 3204293 | doi = 10.1186/1756-6614-4-14 | doi-access = free }}</ref> The adult UL in Japan was last revised to 3,000&nbsp;μg/day in 2015.<ref>{{cite web |title=Overview of Dietary Reference Intakes for Japanese (2015) |publisher=Minister of Health, Labour and Welfare, Japan |url=http://www.mhlw.go.jp/file/06-Seisakujouhou-10900000-Kenkoukyoku/Overview.pdf |access-date=14 March 2022 |archive-date=23 April 2021 |archive-url=https://web.archive.org/web/20210423083531/https://www.mhlw.go.jp/file/06-Seisakujouhou-10900000-Kenkoukyoku/Overview.pdf |url-status=live }}</ref>

After iodine fortification programs such as iodisation of [[Sodium chloride|salt]] have been done, some cases of iodine-induced [[hyperthyroidism]] have been observed (so-called [[Jod-Basedow phenomenon]]). The condition occurs mainly in people above 40 years of age, and the risk is higher when iodine deficiency is high and the first rise in iodine consumption is high.<ref>{{cite journal | vauthors = Wu T, Liu GJ, Li P, Clar C | title = Iodised salt for preventing iodine deficiency disorders | journal = The Cochrane Database of Systematic Reviews | volume = 2010 | issue = 3 | article-number = CD003204 | date = 2002 | pmid = 12137681 | pmc = 9006116 | doi = 10.1002/14651858.CD003204 | veditors = Wu T }}</ref>

===Deficiency===
{{Main|Iodine deficiency}}
In areas where there is little iodine in the diet,<ref name="Dissanayake">{{cite journal| vauthors = Dissanayake CB, Chandrajith R, Tobschall HJ |title = The iodine cycle in the tropical environment – implications on iodine deficiency disorders|journal = International Journal of Environmental Studies|volume = 56 |page= 357| doi = 10.1080/00207239908711210|date = 1999|issue = 3| bibcode=1999IJEnS..56..357D }}</ref> which are remote inland areas and faraway mountainous areas where no iodine rich foods are eaten, [[iodine deficiency]] gives rise to [[hypothyroidism]], symptoms of which are [[Fatigue|extreme fatigue]], [[goitre]], [[Intellectual disability|mental slowing]], [[Depression (mood)|depression]], [[Weight gain|low weight gain]]{{Clarify|reason=What exactly is "low" weight gain?|date=December 2024}}, and [[Hypothermia|low basal body temperatures]].<ref>{{cite book|chapter = Endemic Goiter|title = Endocrinology & metabolism|vauthors = Felig P, Frohman LA|publisher = McGraw-Hill Professional|date = 2001|isbn = 978-0-07-022001-0|chapter-url = https://books.google.com/books?id=AZUUGrp6yUgC&pg=RA1-PA351|access-date = 29 September 2020|archive-date = 12 January 2023|archive-url = https://web.archive.org/web/20230112212836/https://books.google.com/books?id=AZUUGrp6yUgC&pg=RA1-PA351|url-status = live}}</ref> Iodine deficiency is the leading cause of preventable [[intellectual disability]], a result that occurs primarily when babies or small children are rendered [[Hypothyroidism|hypothyroidic]] by no iodine. The addition of iodine to salt has largely destroyed this problem in wealthier areas, but iodine deficiency remains a serious public health problem in poorer areas today.<ref>{{cite web|url =https://www.who.int/nutrition/topics/idd/en/|archive-url =https://web.archive.org/web/20060930020824/http://www.who.int/nutrition/topics/idd/en/|archive-date =30 September 2006|title = Micronutrient deficiency: iodine deficiency disorders|publisher = WHO}}</ref> Iodine deficiency is also a problem in certain areas of all continents of the world. Information processing, fine motor skills, and visual problem solving are normalised by iodine repletion in iodine-deficient people.<ref>{{cite journal | vauthors = Zimmermann MB, Connolly K, Bozo M, Bridson J, Rohner F, Grimci L | title = Iodine supplementation improves cognition in iodine-deficient schoolchildren in Albania: a randomized, controlled, double-blind study | journal = The American Journal of Clinical Nutrition | volume = 83 | issue = 1 | pages = 108–114 | date = January 2006 | pmid = 16400058 | doi = 10.1093/ajcn/83.1.108 | doi-access = free }}</ref>

==Precautions==
===Toxicity===
{{Chembox
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| Section1 = {{Chembox Hazards
| GHSPictograms = {{GHS07}}{{GHS09}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|312|332|315|319|335|372|400}}
| PPhrases = {{P-phrases|261|273|280|305|351|338|314}}<ref>{{cite web|url=https://www.sigmaaldrich.com/catalog/product/sigald/207772|title=Iodine 207772|website=I2|access-date=2 October 2018|archive-date=19 March 2024|archive-url=https://web.archive.org/web/20240319070756/https://www.sigmaaldrich.com/US/en/product/sigald/207772|url-status=live}}</ref>
| NFPA-H = 3
| NFPA-F = 0
| NFPA-R = 0 
| NFPA_ref = <ref>[http://periodictable.com/Elements/053/data.html Technical data for Iodine] {{Webarchive|url=https://web.archive.org/web/20230520014403/https://periodictable.com/Elements/053/data.html |date=20 May 2023 }}. periodictable.com</ref> 
}}
}}
The safe upper limit for iodine consumption is {{val|1000|ul=mcg}}; higher doses may be dangerous for people with certain thyroid issues. However the average Japanese person consumes 5-14 times this much dietary iodine from seaweed ([[kombu]]) with no evident toxicity.<ref name="Patrick2008" />
Liquid [[Povidone-iodine]] (Betadine) trapped against the skin resulted in chemical burns in some rare cases.<ref name="Lowe">{{cite journal | vauthors = Lowe DO, Knowles SR, Weber EA, Railton CJ, Shear NH | title = Povidone-iodine-induced burn: case report and review of the literature | journal = Pharmacotherapy | volume = 26 | issue = 11 | pages = 1641–1645 | date = November 2006 | pmid = 17064209 | doi = 10.1592/phco.26.11.1641 | s2cid = 25708713 }}</ref>

==== Occupational exposure ====
The U.S. [[Occupational Safety and Health Administration]] (OSHA) has set the legal limit ([[Permissible exposure limit]]) for iodine exposure in the workplace at 0.1 ppm (1&nbsp;mg/m<sup>3</sup>) during an 8-hour workday. The [[National Institute for Occupational Safety and Health]] (NIOSH) has set a [[Recommended exposure limit]] (REL) of 0.1&nbsp;ppm (1&nbsp;mg/m<sup>3</sup>) during an 8-hour workday. At levels of 2 ppm, iodine is [[Immediately dangerous to life or health|immediately dangerous to life and health]].<ref>{{cite web|title = CDC - NIOSH Pocket Guide to Chemical Hazards - Iodine|url = https://www.cdc.gov/niosh/npg/npgd0342.html|website = cdc.gov|access-date = 6 November 2015|archive-date = 29 November 2022|archive-url = https://web.archive.org/web/20221129024702/https://www.cdc.gov/niosh/npg/npgd0342.html|url-status = live}}</ref>

====Reaction to iodinated contrast media====
[[Iodinated contrast media]] used in [[computed tomographic]] medical scans may cause adverse reactions, from a mild rash to fatal [[anaphylaxis]], in some people. Previous reactions to the media is the strongest indicator of future issues.<ref name="dermnet">DermNet New Zealand Trust, [http://www.dermnetnz.org/treatments/iodine.html Iodine] {{Webarchive|url=https://web.archive.org/web/20160707101355/http://www.dermnetnz.org/treatments/iodine.html |date=7 July 2016 }}</ref><ref>{{Cite journal |last1=Park |first1=Sae-Jin |last2=Kang |first2=Dong-Yoon |last3=Sohn |first3=Kyoung-Hee |last4=Yoon |first4=Soon-Ho |last5=Lee |first5=Whal |last6=Choi |first6=Young-Hun |last7=Cho |first7=Sang Heon |last8=Kang |first8=Hye-Ryun |date=September 2018 |title=Immediate Mild Reactions to CT with Iodinated Contrast Media: Strategy of Contrast Media Readministration without Corticosteroids |url=http://pubs.rsna.org/doi/10.1148/radiol.2018172524 |journal=Radiology |language=en |volume=288 |issue=3 |pages=710–716 |doi=10.1148/radiol.2018172524 |pmid=29786483 |issn=0033-8419}}</ref> Such reactions have led to the misconception (widely held, even among physicians) that some people are allergic to iodine itself.<ref>{{cite journal | vauthors = Katelaris C |date=2009 |title='Iodine Allergy' label is misleading |journal=Australian Prescriber |volume=32 |pages=125–128 |issue=5 |doi=10.18773/austprescr.2009.061 |doi-access=free }}</ref><ref>{{cite journal | vauthors = Boehm I | title = Seafood allergy and radiocontrast media: are physicians propagating a myth? | journal = The American Journal of Medicine | volume = 121 | issue = 8 | pages = e19 | date = August 2008 | pmid = 18691465 | doi = 10.1016/j.amjmed.2008.03.035 | doi-access = free }}</ref> Improvements in the formulation  of the contrast media has decreased the probability of hypersensitivity reactions.<ref name="ucsf">UCSF Department of Radiology & Biomedical Imaging, [http://www.radiology.ucsf.edu/patient-care/patient-safety/contrast/iodine-allergy Iodine Allergy and Contrast Administration] {{Webarchive|url=https://web.archive.org/web/20210409132913/http://www.radiology.ucsf.edu/patient-care/patient-safety/contrast/iodine-allergy |date=9 April 2021 }}</ref>
{{clear|right}}

===US DEA List I status===
[[Phosphorus]] reduces iodine to [[hydroiodic acid]], which is a reagent effective for reducing [[ephedrine]] and [[pseudoephedrine]] to [[methamphetamine]].<ref>{{cite journal| vauthors = Skinner HF |date = 1990|title = Methamphetamine synthesis via hydriodic acid/red phosphorus reduction of ephedrine|journal = Forensic Science International|volume = 48|issue = 2|pages = 123–134|doi = 10.1016/0379-0738(90)90104-7}}</ref> For this reason, iodine was designated by the United States [[Drug Enforcement Administration]] as a [[DEA list of chemicals#List I chemicals|List I precursor chemical]] under [[Code of Federal Regulations|21 CFR 1310.02]].<ref>{{cite web | url=https://www.deadiversion.usdoj.gov/21cfr/cfr/1310/1310_02.htm | title=PART 1310 - Section 1310.02 Substances covered | access-date=5 December 2019 | archive-date=17 October 2017 | archive-url=https://web.archive.org/web/20171017090223/https://www.deadiversion.usdoj.gov/21cfr/cfr/1310/1310_02.htm }}</ref>

==Notes==
{{Notelist}}

== References ==
{{Reflist}}

==Bibliography==
* {{Greenwood&Earnshaw2nd}}

==External links==
* {{Cite report |url=https://pubs.usgs.gov/publication/mcs2025 |title=Mineral commodity summaries 2025 |date=2025 |publisher=[[U.S. Geological Survey]] |issue=2025 |doi=10.3133/mcs2025 |language=en}} Iodine on page 92.


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[[Category:Iodine| ]]
[[Category:Chemical elements]]
[[Category:Halogens]]
[[Category:Reactive nonmetals]]
[[Category:Diatomic nonmetals]]
[[Category:Dietary minerals]]
[[Category:Oxidizing agents]]
[[Category:Gases with color]]
[[Category:Chemical elements with primitive orthorhombic structure]]