View source for Hydrogen chalcogenide

{{multiple image|align=right|direction=vertical|width=105|image1=Water-3D-balls.png|image2=Hydrogen-sulfide-3D-balls.png|image3=Hydrogen-selenide-3D-balls.png|caption3=Water, hydrogen sulfide, and hydrogen selenide, three simple hydrogen chalcogenides}}'''Hydrogen chalcogenides''' (also '''chalcogen hydrides''' or '''hydrogen chalcides''') are [[binary compounds of hydrogen]] with [[chalcogen]] atoms (elements of group 16: [[oxygen]], [[sulfur]], [[selenium]], [[tellurium]], [[polonium]], and [[livermorium]]). [[Water]], the first [[chemical compound]] in this series, contains one oxygen atom and two [[hydrogen]] atoms, and is the most common compound on the Earth's surface.<ref>{{cite web|url=https://www.cia.gov/the-world-factbook/countries/world/|title=CIA – The world factbook|publisher=[[Central Intelligence Agency]]|access-date=18 August 2016}}</ref>

==Dihydrogen chalcogenides==
The most important series, including water, has the chemical formula H<sub>2</sub>X, with X representing any chalcogen. They are therefore [[triatomic]]. They take on a [[Bent molecular geometry|bent structure]] and as such are [[polar molecule]]s. Water is an essential compound to [[life]] on [[Earth]] today,<ref>{{Cite web | url=https://www.un.org/waterforlifedecade/background.shtml | title=About the International Decade for Action 'Water for Life' 2005-2015}}</ref> covering 70.9% of the planet's surface. The other hydrogen chalcogenides are usually extremely toxic, and have strong unpleasant scents usually resembling rotting eggs or vegetables. Hydrogen sulfide is a common product of decomposition in [[oxygen]]-poor environments and as such is one chemical responsible for the smell of [[flatulence]]. It is also a [[volcanic gas]]. Despite its toxicity, the [[human body]] intentionally produces it in small quantities for use as a [[signaling molecule]].

Water can [[Solubility|dissolve]] the other hydrogen chalcogenides (at least those up to hydrogen telluride), forming [[acid]]ic solutions known as '''hydrochalcogenic acids'''. Although these are weaker acids than the [[hydrohalic acid]]s, they follow a similar trend of acid strength increasing with heavier chalcogens, and also form in a similar way (turning the water into a [[hydronium]] ion H<sub>3</sub>O<sup>+</sup> and the solute into a XH<sup>−</sup> ion). It is unknown if polonium hydride forms an acidic solution in water like its lighter homologues, or if it behaves more like a metal hydride (see also [[hydrogen astatide]]).

{| class="wikitable" style="margin: 1em auto 1em auto"
! Compound !! As [[aqueous solution]] !! [[Chemical formula]] !! Geometry !! [[Acid dissociation constant|p''K''<sub>a</sub>]] !! model
|- style="text-align: center;"
| [[Properties of water|hydrogen oxide]]<br>oxygen hydride<br>water<br>(oxidane) || [[Self-ionization of water|water]] || {{chem2|H2O}} || [[Image:H2O 2D labelled.svg|90px]] || 13.995 || [[File:Water molecule 3D.svg|100px]]
|- style="text-align: center;"
| [[hydrogen sulfide]]<br>sulfur hydride<br>(sulfane) || [[hydrosulfuric acid]] || {{chem2|H2S}} || [[Image:Hydrogen-sulfide-2D-dimensions.svg|90px]] || 7.0 || [[File:Hydrogen-sulfide-3D-vdW.svg|100px]]
|- style="text-align: center;"
| [[hydrogen selenide]]<br>selenium hydride<br>(selane) || [[hydroselenic acid]] || {{chem2|H2Se}} || [[Image:Hydrogen-selenide-2D-dimensions.svg|90px]] || 3.89 || [[File:Hydrogen-selenide-3D-vdW.svg|100px]]
|- style="text-align: center;"
| [[hydrogen telluride]]<br>tellurium hydride<br>(tellane) || [[hydrotelluric acid]] || {{chem2|H2Te}} || [[Image:Hydrogen-telluride-2D-dimensions.svg|90px]] || 2.6 || [[File:Hydrogen-telluride-3D-vdW.svg|100px]]
|- style="text-align: center;"
| [[polonium hydride|hydrogen polonide]]<br>polonium hydride<br>(polane) || [[hydropolonic acid]] || {{chem2|H2Po}} || [[Image:Poloniumwasserstoff.svg|90px]] || ? ||  [[File:Polonium-hydride-3D-vdW.svg|100px]] 
|- style="text-align: center;"
| [[Livermorium#Chemical|hydrogen livermoride]]<br>livermorium hydride<br>(livermorane) || hydrolivermoric acid || {{chem2|H2Lv}} || <!--[[Image:Hydrogen-livermoride-2D-dimensions.svg|90px|link=Special:FilePath/Hydrogen-livermoride-2D-dimensions.svg]]-->|| ? || [[File:Livermorium-hydride-3D-balls.png|frameless|100x100px]]
|}

Some properties of the hydrogen chalcogenides follow:<ref name=Greenwood766>Greenwood and Earnshaw, pp. 766–7</ref>
{| class="wikitable" style="margin: 1em auto 1em auto"
! Property !! {{chem2|H2O}} !! {{chem2|H2S}} !! {{chem2|H2Se}} !! {{chem2|H2Te}} !! {{chem2|H2Po}}
|-
! Melting point (°C)
| 0.0 || −85.6 || −65.7 || −51 || −35.3
|-
! Boiling point (°C)
| 100.0 || −60.3 || −41.3 || −4 || 36.1
|-
! <math>\Delta H^\circ_f \left(\mathrm{kJ \cdot mol}^{-1}\right)</math>
| −285.9 || +20.1 || +73.0 || +99.6 || ?
|-
! Bond angle (H–X–H) (gas)
| 104.45° || 92.1° || 91° || 90° || 90.9° (predicted)<ref name=Sumathi>{{cite journal |journal= Journal of Chemical Physics |year= 1990 |volume= 92 |issue= 11 |pages= 6604–6619 |title= Electronic states and potential energy surfaces of H<sub>2</sub>Te, H<sub>2</sub>Po, and their positive ions |last1= Sumathi |first1= K. |last2= Balasubramanian |first2= K. |doi=10.1063/1.458298|bibcode= 1990JChPh..92.6604S }}</ref>
|-
! Dissociation constant (HX<sup>−</sup>, ''K''<sub>1</sub>)
| 1.8 × 10<sup>−16</sup>
| 1.3 × 10<sup>−7</sup>
| 1.3 × 10<sup>−4</sup>
| 2.3 × 10<sup>−3</sup>
| ?
|-
! Dissociation constant (X<sup>2−</sup>, ''K''<sub>2</sub>)
| 0
| 7.1 × 10<sup>−15</sup>
| 1 × 10<sup>−11</sup>
| 1.6 × 10<sup>−11</sup>
| ?
|}
[[File:Boiling-points_Chalcogen-Halogen.svg|thumb|right|Comparison of the boiling points of the hydrogen chalcogenides and [[hydrogen halide]]s; it can be seen that [[hydrogen fluoride]] similarly exhibits anomalous effects due to hydrogen bonding. [[Ammonia]] also misbehaves similarly.]]
[[File:Mp and bp of H2E (E=O,S,Se,Te,Po) Celsius vs atomic number.svg|thumb|right|Comparison of the melting (blue) and boiling (red) points of the hydrogen chalcogenides. The blue and red lines are least sqares fits for the non-oxygen chalcogenides, showing water should melt at -88&nbsp;°C and boil at -75&nbsp;°C.]]
Many of the anomalous properties of water compared to the rest of the hydrogen chalcogenides may be attributed to significant [[hydrogen bonding]] between hydrogen and oxygen atoms. Some of these properties are the high melting and boiling points (it is a liquid at room temperature), as well as the high [[dielectric constant]] and observable ionic dissociation. Hydrogen bonding in water also results in large values of heat and entropy of vaporisation, surface tension, and viscosity.<ref name=Greenwood623>Greenwood and Earnshaw, p. 623</ref>

The other hydrogen chalcogenides are highly toxic, malodorous gases. Hydrogen sulfide occurs commonly in nature and its properties compared with water reveal a lack of any significant hydrogen bonding.<ref name=Greenwood682>Greenwood and Earnshaw, p. 682</ref> Since they are both gases at STP, hydrogen can be simply burned in the presence of oxygen to form water in a highly [[exothermic]] reaction; such a test can be used in beginner chemistry to test for the gases produced by a reaction as hydrogen will burn with a pop. Water, hydrogen sulfide, and hydrogen selenide may be made by heating their constituent elements together above 350&nbsp;°C, but hydrogen telluride and polonium hydride are not attainable by this method due to their thermal instability; hydrogen telluride decomposes in moisture, in light, and in temperatures above 0&nbsp;°C. Polonium hydride is unstable, and due to the intense radioactivity of polonium (resulting in self-[[radiolysis]] upon formation), only trace quantities may be obtained by treating dilute [[hydrochloric acid]] with polonium-plated [[magnesium]] foil. Its properties are somewhat distinct from the rest of the hydrogen chalcogenides, since polonium is a metal while the other chalcogens are not, and hence this compound is intermediate between a normal hydrogen chalcogenide or [[hydrogen halide]] such as [[hydrogen chloride]], and a metal hydride like [[stannane]]. Like water, the first of the group, polonium hydride is also a liquid at room temperature. Unlike water, however, the strong intermolecular attractions that cause the higher boiling point are [[van der Waals interaction]]s, an effect of the large electron clouds of polonium.<ref name=Greenwood766/>

==Dihydrogen dichalcogenides==
Dihydrogen dichalcogenides have the chemical formula H<sub>2</sub>X<sub>2</sub>, and are generally less stable than the monochalcogenides, commonly decomposing into the monochalcogenide and the chalcogen involved.

The most important of these is [[hydrogen peroxide]], H<sub>2</sub>O<sub>2</sub>, a pale blue, nearly colourless liquid that has a lower volatility than water and a higher density and viscosity. It is important chemically as it can be either oxidised or reduced in solutions of any pH, can readily form peroxometal complexes and peroxoacid complexes, as well as undergoing many proton acid/base reactions. In its less concentrated form hydrogen peroxide has some major household uses, such as a disinfectant or for bleaching hair; [[High-test peroxide|much more concentrated solutions]] are much more dangerous.

{| class="wikitable" style="margin: 1em auto 1em auto"
! Compound || [[Chemical formula]] || Bond length || Model
|-
|style="text-align:center;"| [[hydrogen peroxide]]<br>(dioxidane) ||style="text-align:center;"| H<sub>2</sub>O<sub>2</sub> || [[File:Wasserstoffperoxid.svg|center|90px]]  || [[File:Hydrogen-peroxide-3D-vdW.png|center|100px]] 
|-
|style="text-align:center;"| [[hydrogen disulfide]]<br>(disulfane) ||style="text-align:center;"| H<sub>2</sub>S<sub>2</sub> || [[File:Hydrogen disulfide bonds.png|center|90px]] || [[File:Hydrogen-disulfide-3D-vdW.png|center|100px]] 
|-
|style="text-align:center;"| [[hydrogen diselenide]]<ref>{{cite journal |title= Oxidation of Aqueous Polyselenide Solutions. A Mechanistic Pulse Radiolysis Study |first1= Andreas |last1= Goldbach |first2= Marie-Louise |last2= Saboungi |author-link2=Marie-Louise Saboungi |first3= J. A. |last3= Johnson |first4= Andrew R. |last4= Cook |first5= Dan |last5= Meisel |journal= J. Phys. Chem. A |year= 2000 |volume= 104 |issue= 17 |pages= 4011–4016 |doi= 10.1021/jp994361g |bibcode= 2000JPCA..104.4011G }}</ref><br>(diselane) ||style="text-align:center;"| H<sub>2</sub>Se<sub>2</sub> ||style="text-align:center;"| — || [[File:Hydrogen-diselenide-3D-vdW.png|center|100px]] 
|-
|style="text-align:center;"| [[hydrogen ditelluride]]<ref>{{cite journal |last1=Hop |first1=Cornelis E. C. A. |last2=Medina |first2=Marco A. |date=1994 |title=H<sub>2</sub>Te<sub>2</sub> Is Stable in the Gas Phase |journal=[[Journal of the American Chemical Society]] |volume=1994 |issue=116 |pages=3163–4 |doi=10.1021/ja00086a072}}</ref><br>(ditellane) ||style="text-align:center;"| H<sub>2</sub>Te<sub>2</sub> ||style="text-align:center;"| — || [[File:Hydrogen-ditelluride-3D-vdW.png|center|frameless|100px]]
|}

Some properties of the hydrogen dichalcogenides follow:
{| class="wikitable" style="margin: 1em auto 1em auto"
! Property
! H<sub>2</sub>O<sub>2</sub>
! H<sub>2</sub>S<sub>2</sub>
! H<sub>2</sub>Se<sub>2</sub>
! H<sub>2</sub>Te<sub>2</sub>
|-
! Melting point (°C)
| -0.43
| −89.6
| ?
| ?
|-
! Boiling point (°C)
| 150.2 <sub>(decomposes)</sub>
| 70.7
| ?
| ?
|}

An alternative [[structural isomer]] of the dichalcogenides, in which both hydrogen atoms are bonded to the same chalcogen atom, which is also bonded to the other chalcogen atom, have been examined computationally. These H<sub>2</sub>X<sup>+</sup>–X<sup>–</sup> structures are [[ylide]]s. This isomeric form of hydrogen peroxide, [[oxywater]], has not been synthesized experimentally. The analogous isomer of hydrogen disulfide, [[thiosulfoxide]], has been detected by [[mass spectrometry]] experiments.<ref>{{cite journal |title= Thiosulfoxides (X<sub>2</sub>S=S) and disulfanes (XSSX): first observation of organic thiosulfoxides |first1= Pascal |last1= Gerbaux |first2= Jean-Yves |last2= Salpin |first3= Guy |last3= Bouchoux |first4= Robert |last4= Flammang |journal= International Journal of Mass Spectrometry |volume= 195/196 |year= 2000 |pages= 239–249 |doi= 10.1016/S1387-3806(99)00227-4 |bibcode= 2000IJMSp.195..239G }}</ref>

It is possible for two different chalcogen atoms to share a dichalcogenide, as in [[hydrogen thioperoxide]] (H<sub>2</sub>SO); more well-known compounds of similar description include [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>).

==Higher dihydrogen chalcogenides==
All straight-chain hydrogen chalcogenides follow the [[Homologous series|formula]] {{chem2|H2X_{''n''}|}}.

Higher [[hydrogen polyoxide]]s than {{chem2|H2O2}} are not stable.<ref name=Greenwood633>Greenwood and Earnshaw, pp. 633–8</ref> [[Trioxidane]], with three oxygen atoms, is a transient unstable intermediate in several reactions. The next two in the oxygen series, [[tetraoxidane]] and [[hydrogen pentoxide|pentaoxidane]], have also been synthesized and found to be highly reactive. An alternative structural isomer of trioxidane, in which the two hydrogen atoms are attached to the central oxygen of the three-oxygen chain rather than one on each end, has been examined computationally.<ref name=DobadoJACS1999/>

Beyond {{chem2|H2S}} and {{chem2|H2S2}}, many higher [[polysulfane]]s {{chem2|H2S_{''n''}|}} (''n'' = 3–8) are known as stable compounds.<ref name=RS>R. Steudel "Inorganic Polysulfanes H<sub>2</sub>S<sub>2</sub> with n > 1" in Elemental Sulfur and Sulfur-Rich Compounds II (Topics in Current Chemistry) 2003, Volume 231, pp 99-125. {{doi|10.1007/b13182}}</ref> They feature unbranched sulfur chains, reflecting sulfur's tendency for catenation. Starting with {{chem2|H2S2}}, all known polysulfanes are liquids at room temperature. {{chem2|H2S2}} is colourless while the other polysulfanes are yellow; the colour becomes richer as ''n'' increases, as do the density, viscosity, and boiling point. A table of physical properties is given below.<ref name=Greenwood683>Greenwood and Earnshaw, p. 683</ref>

{| class="wikitable" style="margin: 1em auto 1em auto"
! Compound
! [[Density]] at 20&nbsp;°C (g·cm<sup>−3</sup>)
! [[Vapour pressure]] ([[mmHg]])
! Extrapolated [[boiling point]] ([[°C]])
|-
! {{chem2|H2S}}
| 1.363 (g·dm<sup>−3</sup>)
| 1740 (kPa, 21&nbsp;°C)
| −60
|-
! {{chem2|H2S2}}
| 1.334
| 87.7
| 70
|-
! {{chem2|H2S3}}
| 1.491
| 1.4
| 170
|-
! {{chem2|H2S4}}
| 1.582
| 0.035
| 240
|-
! {{chem2|H2S5}}
| 1.644
| 0.0012
| 285
|-
! {{chem2|H2S6}}
| 1.688
|?
|?
|-
! {{chem2|H2S7}}
| 1.721
|?
|?
|-
! {{chem2|H2S8}}
| 1.747
|?
|?
|}

However, they can easily be oxidised and are all thermally unstable, disproportionating readily to sulfur and hydrogen sulfide, a reaction for which alkali acts as a catalyst:<ref name=Greenwood683/>
:{{chem2|8 H2S_{''n''} → 8 H2S + (''n'' \s 1) S8}}

They also react with [[sulfite]] and [[cyanide]] to produce [[thiosulfate]] and [[thiocyanate]] respectively.<ref name=Greenwood683/>

An alternative structural isomer of the trisulfide, in which the two hydrogen atoms are attached to the central sulfur of the three-sulfur chain rather than one on each end, has been examined computationally.<ref name=DobadoJACS1999>{{cite journal |title= Chemical Bonding in Hypervalent Molecules Revised. 2. Application of the Atoms in Molecules Theory to Y<sub>2</sub>XZ and Y<sub>2</sub>XZ<sub>2</sub> (Y = H, F, CH<sub>3</sub>; X = O, S, Se; Z = O, S) Compounds |first1= J. A. |last1= Dobado |first2= Henar |last2= Martínez-García |first3= José |last3= Molina |first4= Markku R. |last4= Sundberg |journal= J. Am. Chem. Soc. |year= 1999 |volume= 121 |issue= 13 |pages = 3156–3164 |doi= 10.1021/ja9828206 }}</ref> [[Thiosulfurous acid]], a branched isomer of the tetrasulfide, in which the fourth sulfur is bonded to the central sulfur of a linear dihydrogen trisulfide structure ({{chem2|(HS)2S+\sS–}}), has also been examined computationally.<ref>{{cite journal |title= Ab initio study of hypervalent sulfur hydrides as model intermediates in the interconversion reactions of compounds containing sulfur–sulfur bonds |first1= Risto S. |last1= Laitinen |first2= Tapani A. |last2= Pakkanen |first3= Ralf |last3= Steudel |journal= J. Am. Chem. Soc. |year= 1987 |volume= 109 |issue= 3 |pages= 710–714 |doi= 10.1021/ja00237a012 }}</ref> [[Thiosulfuric acid]], in which two sulfur atoms branch off of the central of a linear dihydrogen trisulfide structure has been studied computationally as well.<ref>{{cite journal |title= Hypervalency in sulfur? ''Ab initio'' and DFT studies of the structures of thiosulfate and related sulfur oxyanions |last1= Nishimoto |first1= Akiko |last2= Zhang |first2= Daisy Y. |journal= Sulfur Letters |issue= 5/6 |year= 2003 |volume= 26 |pages= 171–180 |doi= 10.1080/02786110310001622767 |s2cid= 95470892 }}</ref>

Higher polonium hydrides may exist.<ref>{{Cite arXiv |eprint = 1503.08587|last1 = Liu|first1 = Yunxian|title = Phase diagram and superconductivity of polonium hydrides under high pressure|last2 = Duan|first2 = Defang|last3 = Tian|first3 = Fubo|last4 = Li|first4 = Da|last5 = Sha|first5 = Xiaojing|last6 = Zhao|first6 = Zhonglong|last7 = Zhang|first7 = Huadi|last8 = Wu|first8 = Gang|last9 = Yu|first9 = Hongyu|last10 = Liu|first10 = Bingbing|last11 = Cui|first11 = Tian|class = cond-mat.supr-con|year = 2015}}</ref>

==Other hydrogen-chalcogen compounds==
[[File:Heavy-water-3D-vdW.svg|125px|right|thumb|Heavy water]]
Some monohydrogen chalcogenide compounds do exist and others have been studied theoretically. As [[radical (chemistry)|radical compounds]], they are quite unstable. The two simplest are [[hydroxyl radical|hydroxyl]] (HO) and [[hydroperoxyl]] (HO<sub>2</sub>). The compound [[hydrogen ozonide]] (HO<sub>3</sub>) is also known,<ref>{{cite journal |first1= F. |last1= Cacace |first2= G. |last2= de Petris |first3= F. |last3= Pepi |first4= A. |last4= Troiani |title= Experimental Detection of Hydrogen Trioxide |journal= Science |volume= 285 |issue= 5424 |year= 1999 |pages= 81–82 |doi= 10.1126/science.285.5424.81 |pmid= 10390365 }}</ref> along with some of its [[alkali metal]] [[ozonide]] salts are (various MO<sub>3</sub>).<ref name=WibergD>[[#Wiberg2001|Wiberg 2001, p.&nbsp;497]]</ref> The respective sulfur analogue for hydroxyl is [[sulfanyl]] (HS) and HS<sub>2</sub> for hydroperoxyl.
{{multiple image|align=right|direction=vertical|width=105|image1=Hydroxide-3D-balls.png|caption1=HO<sup>−</sup>|image2=Water-3D-balls.png|caption2=H<sub>2</sub>O|image3=Hydronium-3D-balls.png|caption3=H<sub>3</sub>O<sup>+</sup>}}
One or both of the protium atoms in water can be substituted with the [[Isotopes of hydrogen|isotope]] [[deuterium]], yielding respectively [[semiheavy water]] and [[heavy water]], the latter being one of the most famous deuterium compounds. Due to the high difference in density between deuterium and regular [[Hydrogen atom|protium]], heavy water exhibits many anomalous properties. The radioisotope [[tritium]] can also form [[tritiated water]] in much the same way. Another notable deuterium chalcogenide is [[Hydrogen disulfide#Quantum tunneling and its suppression in deuterium disulfide|deuterium disulfide]]. Deuterium telluride (D<sub>2</sub>Te) has slightly higher thermal stability than protium telluride, and has been used experimentally for chemical deposition methods of telluride-based thin films.<ref>Xiao, M. & Gaffney, T. R. Tellurium (Te) Precursors for Making Phase Change Memory Materials. (Google Patents, 2013) (https://www.google.ch/patents/US20130129603)</ref>

Hydrogen shares many properties with the [[halogen]]s; substituting the hydrogen with halogens can result in chalcogen halide compounds such as [[oxygen difluoride]] and [[dichlorine monoxide]], alongside ones that may be impossible with hydrogen such as [[chlorine dioxide]].

===Hydrogen Ions===
One of the most well-known hydrogen chalcogenide ions is the [[hydroxide]] ion, and the related [[hydroxy group|hydroxy]] functional group. The former is present in [[alkali metal]], [[alkaline earth]], and [[Rare-earth element|rare-earth]] hydroxides, formed by reacting the respective metal with water. The hydroxy group appears commonly in organic chemistry, such as within [[alcohol (chemistry)|alcohol]]s. The related [[bisulfide]]/sulfhydryl group appears in [[Sodium hydrosulfide|hydrosulfide salts]] and [[thiol]]s, respectively.

The [[hydronium]] (H<sub>3</sub>O<sup>+</sup>) ion is present in aqueous acidic solutions, including the hydrochalcogenic acids themselves, as well as [[Self-ionization of water|pure water]] alongside hydroxide.

==References ==
{{reflist}}

==Bibliography==
*{{Greenwood&Earnshaw2nd}}
{{Hydrides by group}}

[[Category:Chalcogenides]]
[[Category:Hydrogen compounds]]