Editing Iodine (section)

==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 color 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 melting and boiling points of iodine are the highest among the halogens, conforming to the increasing trend down the group, since 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 halogens. Similarly, iodine is the least volatile of the 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 halogens, iodine has the lowest first [[Ionization energy|ionisation energy]], lowest [[electron affinity]], lowest [[electronegativity]] and lowest reactivity of the 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 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}}
Of the forty known [[isotopes of iodine]], only one occurs in nature, [[Isotopes of iodine|iodine-127]]. The others are radioactive and have half-lives too short to be [[primordial nuclide|primordial]]. As such, iodine is both [[monoisotopic element|monoisotopic]] and [[mononuclidic element|mononuclidic]] and its atomic weight is known to great precision, as it is a constant of nature.<ref name="Greenwood800" />

The longest-lived of the radioactive isotopes of iodine is [[iodine-129]], which has a half-life of 15.7&nbsp;million&nbsp;years, decaying via [[beta decay]] to stable [[xenon]]-129.<ref name="NUBASE">{{NUBASE 2003}}</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}}</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> Traces of iodine-129 still exist today, as it is also a [[cosmogenic nuclide]], formed from [[cosmic ray spallation]] of atmospheric xenon: these traces make up 10<sup>−14</sup> to 10<sup>−10</sup> of all terrestrial iodine. It also occurs from open-air nuclear testing, and is not hazardous because of its very long half-life, the longest of all fission products. At the peak of thermonuclear testing in the 1960s and 1970s, iodine-129 still made up only about 10<sup>−7</sup> of all terrestrial iodine.<ref name="SCOPE50">
[http://www.scopenvironment.org/downloadpubs/scope50 SCOPE 50 - Radioecology after Chernobyl] {{webarchive|url=https://web.archive.org/web/20140513065145/http://www.scopenvironment.org/downloadpubs/scope50/ |date=13 May 2014 }}, the [[Scientific Committee on Problems of the Environment]] (SCOPE), 1993. See table 1.9 in Section 1.4.5.2.</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, no longer than 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]] has a half-life of thirteen 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]] has a half-life of fifty-nine days, 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]], with a half-life of eight days, beta decays to an excited state of stable [[Isotopes of xenon|xenon-131]] that then converts to the ground state by emitting gamma radiation. It 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. As it decays, it may cause damage to the thyroid. 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 usually used against the negative effects of iodine-131 is 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 | s2cid = 30989865 }}</ref> However, iodine-131 may also be used for medicinal purposes in [[radiation therapy]] for this very reason, 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 also 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|url-status=dead}}</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>