Editing Arsenic (section)

== Compounds ==
{{Category see also|Arsenic compounds}}

Compounds of arsenic resemble, in some respects, those of [[phosphorus]], which occupies the same [[Group (periodic table)|group]] (column) of the [[periodic table]]. The most common [[oxidation state]]s for arsenic are: −3 in the [[arsenide]]s, which are alloy-like intermetallic compounds, +3 in the [[arsenite]]s, and +5 in the [[arsenates]] and most organoarsenic compounds. Arsenic also bonds readily to itself as seen in the square {{chem2|As4(3-)}} ions in the mineral [[skutterudite]].<ref>{{cite book |doi = 10.1016/S0080-8784(01)80151-4 |title = Recent Trends in Thermoelectric Materials Research I: Skutterudites: Prospective novel thermoelectrics |date = 2001 |last1 = Uher |first1 = Ctirad |isbn = 978-0-12-752178-7 |volume = 69 |pages = 139–253| series = Semiconductors and Semimetals |chapter = Chapter 5 Skutterudites: Prospective novel thermoelectrics }}</ref> In the +3 [[oxidation state]], arsenic is typically pyramidal owing to the influence of the [[lone pair]] of [[electron]]s.<ref name="Norman" /><!--page 30-->

=== Inorganic compounds ===

One of the simplest arsenic compounds is the trihydride, the highly toxic, flammable, [[pyrophoricity|pyrophoric]] [[arsine]] (AsH<sub>3</sub>). This compound is generally regarded as stable, since at room temperature it decomposes only slowly. At temperatures of 250–300&nbsp;°C decomposition to arsenic and hydrogen is rapid.<ref name="Greenwood557">Greenwood and Earnshaw, pp. 557–558</ref> Several factors, such as [[humidity]], presence of light and certain [[catalyst]]s (namely aluminium) facilitate the rate of decomposition.<ref name="INRS">{{cite web |website=Institut National de Recherche et de Sécurité |title=Fiche toxicologique No. 53: Trihydrure d'arsenic |year = 2000 |url = http://www.inrs.fr/inrs-pub/inrs01.nsf/IntranetObject-accesParReference/FT%2053/$File/ft53.pdf |access-date = 2006-09-06 | archive-url = https://web.archive.org/web/20061126045357/http://www.inrs.fr/inrs-pub/inrs01.nsf/IntranetObject-accesParReference/FT%2053/$FILE/ft53.pdf |archive-date = 26 November 2006 |language=fr}}</ref> It oxidises readily in air to form arsenic trioxide and water, and analogous reactions take place with [[sulfur]] and [[selenium]] instead of [[oxygen]].<ref name="Greenwood557" />

Arsenic forms colorless, odorless, crystalline [[oxide]]s [[Arsenic trioxide|As<sub>2</sub>O<sub>3</sub>]] ("[[white arsenic]]") and [[Arsenic pentoxide|As<sub>2</sub>O<sub>5</sub>]] which are [[hygroscopic]] and readily soluble in water to form acidic solutions. [[Arsenic acid|Arsenic(V) acid]] is a weak acid and its salts, known as [[arsenate]]s,<ref name="Greenwood572" /> are a major source of [[arsenic contamination of groundwater]] in regions with high levels of naturally-occurring arsenic minerals.<ref>{{cite journal |last1=Smedley |first1=P.L |last2=Kinniburgh |first2=D.G |title=A review of the source, behaviour and distribution of arsenic in natural waters |journal=Applied Geochemistry |date=May 2002 |volume=17 |issue=5 |pages=517–568 |doi=10.1016/S0883-2927(02)00018-5 |bibcode=2002ApGC...17..517S |url=http://nora.nerc.ac.uk/id/eprint/12311/1/Abstract.pdf }}</ref> Synthetic arsenates include [[Scheele's Green]] (cupric hydrogen arsenate, acidic copper arsenate), [[calcium arsenate]], and [[lead hydrogen arsenate]]. These three have been used as agricultural [[insecticide]]s and [[poison]]s.

The [[protonation]] steps between the arsenate and arsenic acid are similar to those between [[phosphate]] and [[phosphoric acid]]. Unlike [[phosphorous acid]], [[arsenous acid]] is genuinely tribasic, with the formula As(OH)<sub>3</sub>.<ref name="Greenwood572">Greenwood and Earnshaw, pp. 572–578</ref>

A broad variety of sulfur compounds of arsenic are known. Orpiment ([[arsenic trisulfide|As<sub>2</sub>S<sub>3</sub>]]) and realgar ([[tetraarsenic tetrasulfide|As<sub>4</sub>S<sub>4</sub>]]) are somewhat abundant and were formerly used as painting pigments. In As<sub>4</sub>S<sub>10</sub>, arsenic has a formal oxidation state of +2 in As<sub>4</sub>S<sub>4</sub> which features As-As bonds so that the total covalency of As is still 3.<ref>{{cite web|url=http://www.webelements.com/webelements/compounds/text/As/As4S4-12279902.html|title=Arsenic: arsenic(II) sulfide compound data|access-date=2007-12-10|publisher=WebElements.com| archive-url= https://web.archive.org/web/20071211100733/http://www.webelements.com/webelements/compounds/text/As/As4S4-12279902.html| archive-date= 11 December 2007|url-status = live}}</ref> Both orpiment and realgar, as well as As<sub>4</sub>S<sub>3</sub>, have [[selenium]] analogs; the analogous As<sub>2</sub>Te<sub>3</sub> is known as the mineral [[kalgoorlieite]],<ref>
{{cite web |url=https://www.mindat.org/min-47039.html |title=Kalgoorlieite |date=1993–2017 |website=Mindat |publisher=Hudson Institute of Mineralogy |access-date=2 September 2017}}</ref> and the anion As<sub>2</sub>Te<sup>−</sup> is known as a ligand in [[cobalt]] complexes.<ref name="Greenwood578">Greenwood and Earnshaw, pp. 578–583</ref>

All trihalides of arsenic(III) are well known except the astatide, which is unknown. [[Arsenic pentafluoride]] (AsF<sub>5</sub>) is the only important pentahalide, reflecting the lower stability of the +5 oxidation state; even so, it is a very strong fluorinating and oxidizing agent. (The [[Arsenic pentachloride|pentachloride]] is stable only below −50&nbsp;°C, at which temperature it decomposes to the trichloride, releasing chlorine gas.<ref name="Holl" />)

==== Alloys ====

Arsenic is used as the group 5 element in the [[III-V semiconductor]]s [[gallium arsenide]], [[indium arsenide]], and [[aluminium arsenide]].<ref>{{cite journal |doi = 10.1016/j.taap.2003.10.019 |title = Toxicity of indium arsenide, gallium arsenide, and aluminium gallium arsenide |date = 2004 |last1 = Tanaka |first1 = A. |journal = Toxicology and Applied Pharmacology |volume = 198 |issue = 3 |pages = 405–411 |pmid = 15276420|bibcode = 2004ToxAP.198..405T }}</ref> The valence electron count of GaAs is the same as a pair of Si atoms, but the [[Electronic band structure|band structure]] is completely different which results in distinct bulk properties.<ref>{{cite book |url=https://books.google.com/books?id=pkuuvlNjRtsC&pg=PA1 |title=Light Emitting Silicon for Microphotonics |isbn = 978-3-540-40233-6|last1=Ossicini |first1=Stefano |last2=Pavesi |first2=Lorenzo |last3=Priolo |first3=Francesco |year=2003 |publisher=Springer |access-date=2013-09-27}}</ref> Other arsenic alloys include the II-V semiconductor [[cadmium arsenide]].<ref>{{cite book |doi = 10.1109/SMELEC.1998.781173 |date = 1998 |last1 = Din |first1 = M. B. |last2 = Gould |first2 = R. D. |title = ICSE'98. 1998 IEEE International Conference on Semiconductor Electronics. Proceedings (Cat. No. 98EX187) |chapter = High field conduction mechanism of the evaporated cadmium arsenide thin films |isbn = 978-0-7803-4971-1 |pages = 168–174|s2cid = 110904915 }}</ref>

=== Organoarsenic compounds ===
{{Main|Organoarsenic chemistry}}
[[File:Trimethylarsine-2D.png|left|upright=0.4|thumb|[[Trimethylarsine]]]]

A large variety of organoarsenic compounds are known. Several were developed as [[Chemical weapon|chemical warfare agents]] during World War I, including [[Blister agent|vesicants]] such as [[lewisite]] and vomiting agents such as [[adamsite]].<ref name="Ellison2007">{{cite book|last=Ellison|first=Hank D.|title=Handbook of chemical and biological warfare agents|date=2007|publisher=[[CRC Press]]|isbn=978-0-8493-1434-6}}</ref><ref name="Girard2010">{{cite book|last=Girard|first=James|title=Principles of Environmental Chemistry|date=2010|publisher=Jones & Bartlett Learning|isbn=978-0-7637-5939-1}}</ref><ref name="Somani2001">{{cite book|last=Somani|first=Satu M.|title=Chemical warfare agents: toxicity at low levels|date=2001|publisher=CRC Press|isbn=978-0-8493-0872-7}}</ref> [[Cacodylic acid]], which is of historic and practical interest, arises from the [[methylation]] of arsenic trioxide, a reaction that has no analogy in phosphorus chemistry.  [[Cacodyl]] was the first organometallic compound known (even though arsenic is not a true metal) and was named from the Greek ''κακωδία'' "stink" for its offensive, garlic-like odor; it is very toxic.<ref name="Greenwood584">Greenwood, p. 584</ref>
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