Editing Selenium (section)

===Chalcogen compounds===
[[File:Selenium-dioxide-chain-3D-balls.png|thumb|Structure of the polymer SeO<sub>2</sub>: The (pyramidal) selenium atoms are yellow.|left]]
Selenium forms two [[oxide]]s: [[selenium dioxide]] (SeO<sub>2</sub>) and [[selenium trioxide]] (SeO<sub>3</sub>). Selenium dioxide is formed by combustion of elemental selenium:<ref name="house2008" />
{{block indent|{{chem2|Se + O2 -> SeO2}}}}
It is a [[polymer]]ic solid that forms monomeric SeO<sub>2</sub> molecules in the gas phase. It dissolves in water to form [[selenous acid]], H<sub>2</sub>SeO<sub>3</sub>. Selenous acid can also be made directly by oxidizing elemental selenium with [[nitric acid]]:<ref name="wiberg_holleman">{{cite book |title=Inorganic chemistry |last1=Wiberg |first1=Egon |last2=Wiberg |first2=Nils |last3=Holleman |first3=Arnold Frederick |publisher=Academic Press |year=2001 |location=San Diego |isbn=978-0-12-352651-9 |page=583}}</ref>

{{block indent|{{chem2|3 Se + 4 HNO3 + H2O -> 3 H2SeO3 + 4 NO}}}}
Unlike sulfur, which forms a stable [[sulfur trioxide|trioxide]], selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185&nbsp;°C:<ref name="house2008" /><ref name="wiberg_holleman" />

{{block indent|{{chem2| 2 SeO3 -> 2 SeO2 + O2}} (ΔH {{=}} −54&nbsp;kJ/mol)}}
Selenium trioxide is produced in the laboratory by the reaction of [[anhydrous]] [[potassium selenate]] (K<sub>2</sub>SeO<sub>4</sub>) and sulfur trioxide (SO<sub>3</sub>).<ref>{{Greenwood&Earnshaw|page=780}}</ref>

[[Salt (chemistry)|Salts]] of selenous acid are called selenites. These include [[silver selenite]] (Ag<sub>2</sub>SeO<sub>3</sub>) and [[sodium selenite]] (Na<sub>2</sub>SeO<sub>3</sub>).

[[Hydrogen sulfide]] reacts with aqueous selenous acid to produce [[selenium disulfide]]:
{{block indent|{{chem2|H2SeO3 + 2 H2S -> SeS2 + 3 H2O}}}}

Selenium disulfide consists of 8-membered rings. It has an approximate composition of SeS<sub>2</sub>, with individual rings varying in composition, such as Se<sub>4</sub>S<sub>4</sub> and Se<sub>2</sub>S<sub>6</sub>. Selenium disulfide has been used in shampoo as an anti[[dandruff]] agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in [[fireworks]].<ref name="wiberg_holleman" />

Selenium trioxide may be synthesized by dehydrating [[selenic acid]], H<sub>2</sub>SeO<sub>4</sub>, which is itself produced by the oxidation of selenium dioxide with [[hydrogen peroxide]]:<ref>{{Cite book |doi=10.1002/9780470132517.ch9 |last1=Seppelt |first1=K. |last2=Desmarteau |first2=Darryl D. |chapter=Selenonyl Difluoride |title=Inorganic Syntheses |year=1980 |volume=20 |pages=36–38 |isbn=978-0-471-07715-2 }} The report describes the synthesis of selenic acid.</ref>
{{block indent|{{chem2|SeO2 + H2O2 -> H2SeO4}}}}

Hot, concentrated selenic acid reacts with gold to form gold(III) selenate.<ref>{{Cite journal |last1=Lenher |first1=V. |date=April 1902 |title=Action of selenic acid on gold |journal=Journal of the American Chemical Society |volume=24 |issue=4 |pages=354–355 |doi=10.1021/ja02018a005 |bibcode=1902JAChS..24..354L |url=https://zenodo.org/record/1428902}}</ref>