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Hydrogen chalcogenide
Hydrogen chalcogenide
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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.[1]

Dihydrogen chalcogenides

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The most important series, including water, has the chemical formula H2X, with X representing any chalcogen. They are therefore triatomic. They take on a bent structure and as such are polar molecules. Water is an essential compound to life on Earth today,[2] 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 dissolve the other hydrogen chalcogenides (at least those up to hydrogen telluride), forming acidic solutions known as hydrochalcogenic acids. Although these are weaker acids than the hydrohalic acids, 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 H3O+ and the solute into a XH− 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).

Compound As aqueous solution Chemical formula Geometry pKa model
hydrogen oxide
oxygen hydride
water
(oxidane)		 water H2O 13.995
hydrogen sulfide
sulfur hydride
(sulfane)		 hydrosulfuric acid H2S 7.0
hydrogen selenide
selenium hydride
(selane)		 hydroselenic acid H2Se 3.89
hydrogen telluride
tellurium hydride
(tellane)		 hydrotelluric acid H2Te 2.6
hydrogen polonide
polonium hydride
(polane)		 hydropolonic acid H2Po ?
hydrogen livermoride[3]
livermorium hydride
(livermorane)		 hydrolivermoric acid H2Lv ?

Some properties of the hydrogen chalcogenides follow:[4]

Property H2O H2S H2Se H2Te 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
−285.9 +20.1 +73.0 +99.6 ?
Bond angle (H–X–H) (gas) 104.45° 92.1° 91° 90° 90.9° (predicted)[5]
Dissociation constant (HX−, K1) 1.8 × 10−16 1.3 × 10−7 1.3 × 10−4 2.3 × 10−3 ?
Dissociation constant (X2−, K2) 0 7.1 × 10−15 1 × 10−11 1.6 × 10−11 ?
Comparison of the boiling points of the hydrogen chalcogenides and hydrogen halides; it can be seen that hydrogen fluoride similarly exhibits anomalous effects due to hydrogen bonding. Ammonia also misbehaves similarly.
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 °C and boil at −75 °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.[6]

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.[7] 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 °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 °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 interactions, an effect of the large electron clouds of polonium.[4]

Dihydrogen dichalcogenides

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Dihydrogen dichalcogenides have the chemical formula H2X2, 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, H2O2, 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; much more concentrated solutions are much more dangerous.

Compound Chemical formula Bond length Model
hydrogen peroxide
(dioxidane)		 H2O2
hydrogen disulfide
(disulfane)		 H2S2
hydrogen diselenide[8]
(diselane)		 H2Se2 —
hydrogen ditelluride[9]
(ditellane)		 H2Te2 —

Some properties of the hydrogen dichalcogenides follow:

Property H2O2 H2S2 H2Se2 H2Te2
Melting point (°C) −0.43 −89.6 ? ?
Boiling point (°C) 150.2 (decomposes) 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 H2X+–X– structures are ylides. 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.[10]

It is possible for two different chalcogen atoms to share a dichalcogenide, as in hydrogen thioperoxide (H2SO); more well-known compounds of similar description include sulfuric acid (H2SO4).

Higher dihydrogen chalcogenides

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All straight-chain hydrogen chalcogenides follow the formula H2Xn.

Higher hydrogen polyoxides than H2O2 are not stable.[11] Trioxidane, with three oxygen atoms, is a transient unstable intermediate in several reactions. The next two in the oxygen series, tetraoxidane and 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.[12]

Beyond H2S and H2S2, many higher polysulfanes H2Sn (n = 3–8) are known as stable compounds.[13] They feature unbranched sulfur chains, reflecting sulfur's tendency for catenation. Starting with H2S2, all known polysulfanes are liquids at room temperature. 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.[14]

Compound Density at 20 °C (g·cm−3) Vapour pressure (mmHg) Extrapolated boiling point (°C)
H2S 1.363 (g·dm−3) 1740 (kPa, 21 °C) −60
H2S2 1.334 87.7 70
H2S3 1.491 1.4 170
H2S4 1.582 0.035 240
H2S5 1.644 0.0012 285
H2S6 1.688 ? ?
H2S7 1.721 ? ?
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:[14]

8 H2Sn → 8 H2S + (n − 1) S8

They also react with sulfite and cyanide to produce thiosulfate and thiocyanate respectively.[14]

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.[12] 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 ((HS)2S+−S−), has also been examined computationally.[15] Thiosulfuric acid, in which two sulfur atoms branch off of the central of a linear dihydrogen trisulfide structure has been studied computationally as well.[16]

Higher polonium hydrides may exist.[17]

Other hydrogen-chalcogen compounds

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Heavy water

Some monohydrogen chalcogenide compounds do exist and others have been studied theoretically. As radical compounds, they are quite unstable. The two simplest are hydroxyl (HO) and hydroperoxyl (HO2). The compound hydrogen ozonide (HO3) is also known,[18] along with some of its alkali metal ozonide salts are (various MO3).[19] The respective sulfur analogue for hydroxyl is sulfanyl (HS) and HS2 for hydroperoxyl.

HO−
H2O
H3O+

One or both of the protium atoms in water can be substituted with the 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 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 deuterium disulfide. Deuterium telluride (D2Te) has slightly higher thermal stability than protium telluride, and has been used experimentally for chemical deposition methods of telluride-based thin films.[20]

Hydrogen shares many properties with the halogens; 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

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One of the most well-known hydrogen chalcogenide ions is the hydroxide ion, and the related hydroxy functional group. The former is present in alkali metal, alkaline earth, and rare-earth hydroxides, formed by reacting the respective metal with water. The hydroxy group appears commonly in organic chemistry, such as within alcohols. The related bisulfide/sulfhydryl group appears in hydrosulfide salts and thiols, respectively.

The hydronium (H3O+) ion is present in aqueous acidic solutions, including the hydrochalcogenic acids themselves, as well as pure water alongside hydroxide.

References

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Bibliography

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Hydrogen chalcogenide

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Hydrogen chalcogenides, also known as hydrides, are binary compounds formed between hydrogen and the elements of group 16 in the periodic table, including oxygen, , , , and . The primary examples are the dihydrides H₂O (water), H₂S (), H₂Se (), H₂Te (), and H₂Po (hydrogen polonide). These colorless, toxic gases (except for H₂O) exhibit a bent, V-shaped due to the lone pairs on the central atom, with bond angles decreasing from approximately 104.5° in H₂O to 89.5° in H₂Te as atomic size increases down the group. The physical of chalcogenides show distinct trends. points generally increase with down the group, from -60.7°C for H₂S, -41.3°C for H₂Se, and -2°C for H₂Te, but H₂O has an anomalously high of 100°C due to strong intermolecular hydrogen bonding. Similarly, melting points rise from -85.5°C for H₂S to -49°C for H₂Te. Acidity strengthens down the group as the H–E bond weakens and the E–H bond polarity decreases; for the first dissociation, pKa values are approximately 15.7 for H₂O, 7.0 for H₂S, 3.9 for H₂Se, and 2.6 for H₂Te. Beyond the dihydrides, sulfur forms higher-order hydrides known as sulfanes (H₂Sₙ, where n = 2–8), which are unstable, viscous yellow liquids used in specialized syntheses. The non-oxygen chalcogenides (H₂S, H₂Se, H₂Te) are prepared industrially by reacting metal chalcogenides with acids, such as Al₂S₃ + 6 HCl → 3 H₂S + 2 AlCl₃, and are notable for their foul odors and high , with H₂S being particularly dangerous at concentrations above 100 ppm due to its interference with . H₂Po is highly unstable and radioactive, decomposing rapidly. These compounds play key roles in , , and as precursors in materials.

General Characteristics

Definition and Classification

Hydrogen chalcogenides, also known as chalcogen hydrides, are binary compounds composed of and one or more atoms from the elements of group 16 in the periodic table. The s include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), polonium (Po), and the synthetic element livermorium (Lv). The term "" derives from words chalkos (meaning or ) and genes (meaning former or producer), highlighting the historical association of these elements with formation; it was first proposed in 1932 by German chemist Werner . These elements exhibit a clear trend of increasing metallic character down the group, transitioning from the nonmetals oxygen and at the top to the metalloids and , and finally to the ./09%3A_Group_16/9.01%3A_The_Group_16_Elements-_The_Chalcogens) , being highly radioactive and synthetic, has limited experimental data but is predicted to follow this trend toward greater metallicity. Hydrogen chalcogenides are primarily classified based on the number of atoms in the molecule and the nature of the compound. The principal series consists of dihydrogen monochalcogenides with the general formula H2XH_2X, where X denotes a atom; examples include (H2OH_2O) and (H2SH_2S). Further classification encompasses dihydrogen dichalcogenides (H2X2H_2X_2), such as dihydrogen disulfide (H2S2H_2S_2), and higher dihydrogen polychalcogenides (H2XnH_2X_n, where n>2n > 2), known as polysulfanes or analogous polyselenanes/polytelluranes for heavier chalcogens (e.g., H2SnH_2S_n up to n=35n=35). Additional categories include ionic species like hydrosulfide ions (HS−HS^-) and radicals such as hydroxyl (OH⋅OH^\cdot). This encyclopedia entry focuses exclusively on purely binary hydrogen- compounds, excluding ternary or more complex species such as (H2SO4H_2SO_4). The stability of hydrogen chalcogenides tends to decrease with increasing of the chalcogen.

Bonding and Molecular Structure

The hydrogen chalcogenides, general formula H₂E (where E is O, S, Se, or Te), feature bent molecular geometries as described by the Valence Shell Electron Pair Repulsion (VSEPR) theory. Each central chalcogen atom contributes six valence electrons, forming two single bonds to hydrogen atoms while retaining two lone pairs of electrons. This arrangement corresponds to an AX₂E₂ notation, yielding a tetrahedral electron-pair geometry but a nonlinear molecular shape due to the repulsion between the lone pairs and bonding pairs, which compresses the H–E–H angle below the ideal tetrahedral value of 109.5°./09%3A_Molecular_Geometry_and_Covalent_Bonding_Models/9.02%3A_VSEPR_-_Molecular_Geometry) The H–E–H bond angles decrease progressively down the group, from 104.45° in H₂O to 92.1° in H₂S, 91° in H₂Se, and 89.5° in H₂Te. This trend results from the increasing atomic size and lower of the heavier chalcogens, which lead to greater s-character in the bonding orbitals and reduced hybridization effects, allowing the bond angles to approach the 90° expected for pure p-orbital overlap.65.pdf) Concurrently, the E–H bond lengths increase with the size of the central atom: 95.8 pm for O–H, 134 pm for S–H, 146 pm for Se–H, and 170 pm for Te–H, reflecting weaker orbital overlap and longer covalent bonds in the heavier congeners. The central chalcogen atoms adopt sp³ hybridization in these molecules, forming four equivalent sp³ hybrid orbitals that accommodate the two σ-bonding pairs to and the two lone pairs. This hybridization model explains the tetrahedral electron arrangement and the resulting bent structure, with the lone pairs occupying hybrid orbitals that exert stronger repulsion on the bonding pairs./10%3A_Bonding_in_Polyatomic_Molecules/10.02%3A_Hybrid_Orbitals_in_Water) Intermolecular forces vary significantly across the series. In H₂O, strong hydrogen bonding predominates due to oxygen's high , enabling effective interactions between the partially positive atoms and lone pairs on adjacent molecules. In contrast, the heavier H₂S, H₂Se, and H₂Te rely mainly on – forces and dispersion forces, as the decreased of S, Se, and Te diminishes the polarity of the E–H bonds and prevents significant hydrogen bonding. Bond dissociation energies for the E–H bonds also decrease down the group, indicating progressively weaker bonds: approximately 463 kJ/mol for H–O, 366 kJ/mol for H–S, with further reductions for H–Se and H–Te. These values reflect the diminishing bond strength due to poorer overlap between the 1s orbital and the larger, more diffuse valence orbitals of the heavier chalcogens.

Dihydrogen Monochalcogenides

Water and Its Unique Properties

Water (H₂O) exhibits distinctive physical properties that set it apart from other hydrogen chalcogenides. At standard , it melts at 0 °C and boils at 100 °C, allowing it to exist as a over a wide range relevant to Earth's . Its reaches a maximum of 1 g/cm³ at 4 °C, after which it anomalously expands upon further cooling and freezing, resulting in being less dense than water and thus floating on its surface. This expansion, which increases volume by about 9% during freezing, arises from the open tetrahedral structure formed by hydrogen bonds in the solid phase. The unique stem largely from its extensive bonding network, where each molecule can form up to four bonds with neighboring molecules. This network contributes to water's high of 4.184 J/g·K, enabling it to absorb significant with minimal change and thus moderate environmental temperatures. Similarly, the cohesive forces from bonds produce a high of approximately 72 mN/m at 25 °C, allowing phenomena such as and the support of small objects on the surface. Chemically, water displays amphoteric behavior, capable of acting as either an or a base depending on the reactant. It undergoes autoionization to produce (H₃O⁺) and (OH⁻) s, with the product K_w equal to 1.0 × 10^{-14} at 25 °C, establishing the scale for aqueous solutions. The itself is a with a pK_a of approximately -1.7, representing the lower limit of acidity in . Water participates in key reactions, including oxidation to molecular oxygen (O₂) during processes like and , and reduction to hydrogen gas (H₂) in electrolytic . In reactions, water molecules cleave chemical bonds in larger compounds, such as esters or amides, by donating a hydroxyl group and accepting a proton, facilitating metabolic breakdown in biological systems. Biologically, serves as the universal for , enabling the dissolution and transport of nutrients, ions, and metabolites essential for cellular processes, and comprising –90% of the mass of living organisms. Environmentally, it covers about 71% of Earth's surface, primarily as oceans, and drives the global through , , , and runoff, regulating and sustaining ecosystems. Water exists in various isotopic forms, including light water (¹H₂¹⁶O) and heavier isotopologues like deuterium oxide (²H₂O, or ) and those with ¹⁸O, which influence physical properties slightly and are detailed further in discussions of isotopologues.

Hydrogen Sulfide, Selenide, Telluride, and Heavier Analogues

The dihydrogen monochalcogenides beyond , namely (H₂S), hydrogen selenide (H₂Se), and (H₂Te), exhibit physical properties that trend with increasing of the atom, primarily due to enhanced molecular and weaker intermolecular forces compared to hydrogen bonding in water. These compounds are colorless gases at standard conditions, with points rising down the group as van der Waals interactions strengthen with larger, more polarizable electron clouds. The following table summarizes key physical properties:
CompoundBoiling Point (°C)Molecular Weight (g/mol)
H₂S-60.334.08
H₂Se-41.380.98
H₂Te-2.0129.62
These values reflect the increasing boiling points, which facilitate phase transitions at higher temperatures for heavier analogues. In aqueous solutions, H₂S, H₂Se, and H₂Te act as weak diprotic acids, with increasing down the group due to progressively weaker E–H bonds (where E is the ) and greater stability of the conjugate bases. The first dissociation constants show this trend clearly: pK_{a1} for H₂S is 7.0, for H₂Se is 3.9, and for H₂Te is 2.6. This enhancement in acidity arises from the decreasing and as the chalcogen size increases, making proton release more favorable. These compounds are highly toxic, with H₂S notorious for its characteristic "rotten egg" odor detectable at low concentrations (around 0.00047 ppm), though olfactory fatigue can occur rapidly. Exposure to H₂S at concentrations above 500 ppm is lethal within minutes due to respiratory paralysis and . H₂Se and H₂Te exhibit similar toxicity profiles, causing severe to the eyes, , and mucous membranes, with H₂Se having an occupational exposure limit of 0.05 ppm and H₂Te being acutely toxic via , leading to and . All three pose significant hazards in industrial settings, such as and chemical . As reducing agents, H₂S, H₂Se, and H₂Te readily donate electrons or , reacting with metals and metal ions to form insoluble chalcogenides; for example, they precipitate sulfides, selenides, and tellurides from aqueous solutions of metal salts, a principle used in qualitative inorganic analysis. Upon combustion in air, they oxidize to produce chalcogen dioxides and : H₂S burns to SO₂, while analogous reactions yield SeO₂ and TeO₂ for the heavier compounds. These reactions highlight their utility in synthetic chemistry but also underscore flammability risks. Heavier analogues like hydrogen polonide (H₂Po) are extremely unstable and radioactive, decomposing rapidly due to the polonium atom's and weak Po–H bonds, with possible metallic hydride-like behavior under . Experimental data remain limited owing to polonium's scarcity and radioactivity, but post-2020 computational predictions estimate the average Po–H at approximately 250 kJ/mol, indicating even greater instability than H₂Te. Hydrogen livermoride (H₂Lv), the theoretical compound with , is predicted to be highly unstable, with relativistic effects destabilizing the Lv–H bonds and rendering synthesis infeasible with current technology; no experimental observations exist, and models suggest decomposition pathways dominated by nuclear instability.

Dihydrogen Oligochalcogenides

Dihydrogen Dichalcogenides

Dihydrogen dichalcogenides are compounds with the general formula H₂X₂, where X represents a atom (O, S, Se, or Te), featuring a central X–X between two XH groups. These molecules adopt a skewed or twisted conformation to minimize steric repulsion and lone-pair interactions, with bond lengths increasing down the group due to larger atomic radii. Unlike the monochalcogenides H₂X, which are generally stable, H₂X₂ compounds exhibit reduced thermal and , except for H₂O₂, primarily because the X–X bond weakens relative to the stability of the decomposition products H₂X and X. Hydrogen peroxide (H₂O₂) is the most stable and well-characterized member of this series. Its structure features a skewed conformation with a central of 146 pm and O–H bond lengths of approximately 96 pm, resulting from the repulsion between lone pairs on adjacent oxygen atoms. It has a of -0.4 °C and a of 150 °C, though it decomposes catalytically above 60 °C to yield and oxygen via the 2H₂O₂ → 2H₂O + O₂. The O–O bond dissociation energy is 146 kJ/mol, significantly weaker than the O=O bond in O₂ (498 kJ/mol), contributing to its reactivity. H₂O₂ is widely used as a bleaching agent in textiles and paper, a in medical and applications, and as a high-energy oxidizer in rocket propulsion systems due to its high oxygen content and clean products. Disulfane (H₂S₂), the sulfur analogue, is a pale yellow oil with a camphor-like , but it is highly unstable and decomposes above -60 °C into and elemental . Its synthesis typically involves the reaction of with sulfur monochloride (H₂S + SCl₂ → H₂S₂ + 2HCl), often conducted at low temperatures to isolate the product. The S–S is about 206 pm, and the bond dissociation energy is approximately 266 kJ/mol, stronger than the O–O bond in H₂O₂; however, the overall molecule remains unstable due to weak S–H bonds and facile rearrangement to polysulfanes. Disulfane finds limited use in laboratory synthesis of sulfur-containing compounds but lacks commercial applications owing to its instability. The heavier analogues, H₂Se₂ and H₂Te₂, are even less stable, with limited experimental data available due to their tendency to decompose rapidly at . Hydrogen diselenide (H₂Se₂) appears as a reddish liquid and is explosive upon heating or shock, dissociating into and . Hydrogen ditelluride (H₂Te₂) is similarly unstable, adopting a non-planar twisted structure but decomposing almost immediately to and . Stability decreases down the group, correlating with increasing X–X bond lengths (Se–Se ~232 pm, Te–Te ~266 pm) and progressively weaker overall molecular cohesion despite stronger individual X–X bonds in heavier elements. These compounds are primarily of interest in specialized laboratory syntheses for studying chemistry.

Higher Dihydrogen Polychalcogenides

Higher dihydrogen polychalcogenides encompass neutral molecular compounds of the general formula H₂Xₙ, where X represents a atom (oxygen, sulfur, , or ) and n ranges from 3 to 8 or higher. These species extend the chain length beyond the dichalcogenides, featuring catenated atoms bonded to terminal hydrogens, and exhibit varying degrees of stability influenced by the chalcogen's atomic size and . Polysulfanes (H₂Sₙ) are the most prominent and relatively stable members of this class, with chains up to n=11 observed in experimental studies under controlled conditions, though they generally decompose to (H₂S) and elemental (S₈ or S⁰). Synthesis of polysulfanes typically involves acid hydrolysis of salts (S₂O₃²⁻), which generates short-chain H₂Sₙ species through and sulfur-sulfur bond cleavage, or controlled reactions between H₂S and sulfur halides like S₂Cl₂. Notable examples include trisulfane (H₂S₃), a pale liquid with a of 1.495 g/cm³ at 15°C and a of approximately 170°C, and hexa-sulfane (H₂S₆), described as a viscous ; as chain length increases, the color intensifies from to deep red due to enhanced light absorption by the extended chain. These compounds are thermally labile, with stability peaking around n=6 before decomposition accelerates, often yielding H₂S and cyclic S₈. Oxygen-based analogues, such as (H₂O₃, also known as dihydrogen trioxide), are markedly unstable, decomposing rapidly via O-O bond scission to form and or , with half-lives on the order of milliseconds in aqueous media but extending to minutes in organic solvents. Higher oxygen polyoxides (n>3) are exceedingly rare and transient, often generated transiently in peroxy radical condensates. Photolysis of or the peroxone process ( with H₂O₂) serves as a key synthetic route for H₂O₃, stabilizing it briefly in low-temperature matrices for spectroscopic analysis. For heavier chalcogens, and variants like H₂Se₃ (hydrogen triselenide) and H₂Te₃ (hydrogen tritelluride) exist primarily as short-lived intermediates in aqueous or gaseous phases, prone to rapid decomposition into H₂X (X=Se, Te) and elemental chalcogen, with dissociation constants suggesting even greater instability than their sulfur counterparts. Characterization remains limited, relying on computational models and indirect detection in polyselenide solutions, as direct isolation is challenging due to their fleeting nature. Post-2020 studies have computationally modeled longer H₂Teₙ chains, highlighting their potential electronic band structures for applications, though experimental realization lags.

Other Hydrogen-Chalcogen Compounds

Ions and Radicals

Hydrogen chalcogenides form various ions through and equilibria in aqueous solutions. For , the autoprotolysis reaction 2H2O⇌H3O++OH−2\mathrm{H_2O} \rightleftharpoons \mathrm{H_3O^+} + \mathrm{OH^-} produces the hydronium ion (H3O+\mathrm{H_3O^+}), which predominates in acidic conditions, and the hydroxide ion (OH−\mathrm{OH^-}), which is the key basic species derived from H2O\mathrm{H_2O}. Similarly, (H2S\mathrm{H_2S}) dissociates stepwise as H2S⇌HS−+H+\mathrm{H_2S} \rightleftharpoons \mathrm{HS^-} + \mathrm{H^+} and HS−⇌S2−+H+\mathrm{HS^-} \rightleftharpoons \mathrm{S^{2-}} + \mathrm{H^+}, yielding the hydrosulfide (HS−\mathrm{HS^-}) and (S2−\mathrm{S^{2-}}) ions, while hydrogen selenide (H2Se\mathrm{H_2Se}) forms HSe−\mathrm{HSe^-} via H2Se⇌HSe−+H+\mathrm{H_2Se} \rightleftharpoons \mathrm{HSe^-} + \mathrm{H^+}. These ions are prevalent in geochemical and biological contexts, with H3O+\mathrm{H_3O^+}, OH−\mathrm{OH^-}, HS−\mathrm{HS^-}, S2−\mathrm{S^{2-}}, and HSe−\mathrm{HSe^-} representing the primary charged species from the lighter hydrogen chalcogenides. The acidity of the conjugate acids governs ion stability and prevalence. The second dissociation constant for H2S\mathrm{H_2S} (pKa2_a2 of HS−\mathrm{HS^-}) is approximately 13 at 25°C, indicating HS−\mathrm{HS^-} is a moderately , while for H2Se\mathrm{H_2Se}, the pKa2_a2 of HSe−\mathrm{HSe^-} is approximately 11, making HSe−\mathrm{HSe^-} a stronger base than S2−\mathrm{S^{2-}} but weaker than OH−\mathrm{OH^-}. Basicity of these chalcogenide s decreases down the group from OH−>HS−>HSe−\mathrm{OH^-} > \mathrm{HS^-} > \mathrm{HSe^-}, as larger central atoms (O to Se) result in more diffuse lone pairs and reduced , weakening their ability to accept protons. This trend aligns with increasing acid strength of the parent H2X\mathrm{H_2X} compounds down the group, influencing speciation in natural waters. Radicals derived from hydrogen chalcogenides, such as the hydroxyl (⋅OH\cdot\mathrm{OH}) and hydrosulfide (⋅HS\cdot\mathrm{HS}) species, exhibit high reactivity due to unpaired electrons. The hydroxyl radical is generated in the atmosphere via \mathrm{O_3 + h\nu \rightarrow O(^1D) + O_2 followed by O(1D)+H2O→2⋅OH\mathrm{O(^1D) + H_2O \rightarrow 2\cdot\mathrm{OH}}, and it plays a central role in ozone depletion by initiating halogen radical chains on polar stratospheric clouds, where photo-generated ⋅OH\cdot\mathrm{OH} facilitates Cl and Br release for catalytic O3\mathrm{O_3} destruction. In aqueous environments, ⋅OH\cdot\mathrm{OH} has an ultrashort lifetime of approximately 10−910^{-9} s, reacting rapidly with most organic and inorganic species. The hydrosulfide radical (⋅HS\cdot\mathrm{HS}), formed in combustion processes like H2S+⋅H⇌⋅HS+H2\mathrm{H_2S} + \cdot\mathrm{H} \rightleftharpoons \cdot\mathrm{HS} + \mathrm{H_2}, is key in modeling sulfur chemistry during hydrogen sulfide oxidation, influencing flame propagation and pollutant formation in fuel-rich conditions. In , HS−\mathrm{HS^-} is significant in systems, where it dominates speciation in alkaline, reduced fluids emanating from mid-ocean ridges, facilitating metal precipitation and supporting chemosynthetic microbial communities. Atmospheric ⋅OH\cdot\mathrm{OH}, meanwhile, acts as a sink for ozone-depleting substances by oxidizing volatile organics and halocarbons, though its indirect role in stratospheric cycles amplifies depletion events. These ions and radicals highlight the dynamic chemistry of hydrogen chalcogenides across environmental scales.

Isotopologues and Isotopes

Hydrogen chalcogenides exhibit isotopic variations primarily through substitutions of hydrogen and chalcogen atoms, leading to isotopologues with distinct physical and chemical properties. These isotopes influence molecular behavior due to mass differences, affecting bond strengths, reaction rates, and spectroscopic signatures, which are leveraged in scientific applications. Deuterated analogs, such as heavy water (D₂O), replace protium with deuterium, resulting in a higher melting point of 3.8 °C and boiling point of 101.4 °C compared to ordinary water. D₂O also possesses a greater density of 1.105 g/cm³ at 20 °C, attributed to the increased atomic mass of deuterium strengthening hydrogen bonding. These properties arise from the heavier isotope's reduced zero-point energy, which stabilizes the molecule. Tritiated water (T₂O), incorporating the radioactive tritium isotope (³H), is a β-emitter with a half-life of 12.32 years, decaying to helium-3; its chemical behavior mirrors H₂O but introduces radiological hazards due to β-particle emission. Chalcogen isotope substitutions, such as ³⁴S in H₂³⁴S, enable tracing of sulfur cycles in environmental and geological contexts. The δ³⁴S ratio, defined as the deviation in ³⁴S/³²S from a standard, reveals fractionations from microbial processes or atmospheric reactions, aiding paleoclimate reconstructions of ancient sulfur budgets and conditions. Property differences in isotopologues manifest as kinetic isotope effects; for instance, reactions involving O-H bonds in D₂O proceed slower due to the higher mass of , exemplified by reduced rates of H/D exchange in proton transfer processes, where bonds break more sluggishly. Applications of these isotopologues are diverse. D₂O serves as a in CANDU reactors, slowing neutrons to sustain fission in fuel without enrichment, due to its low neutron absorption cross-section. In biochemistry, D₂O enables for tracking metabolic pathways, such as and hydrogen exchange in , providing insights into cellular dynamics without altering molecular structure significantly. H₂¹⁸O, enriched with the stable isotope, traces the global oxygen cycle by monitoring fractionation during and , revealing hydrological processes and climate variability. For heavier chalcogens like and , no significant isotopic data exist for their hydrogen compounds owing to the extreme and instability of these elements, limiting synthesis and study.

References

  1. https://link.springer.com/chapter/10.1007/978-3-031-06878-2_12
  2. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Chemistry_of_the_Main_Group_Elements_%28Barron%29/09%253A_Group_16/9.07%253A_Chalconide_Hydrides
  3. https://www2.chem.wisc.edu/areas/reich/pkatable/pKa_compilation-1-Williams.pdf
  4. https://pubs.rsc.org/en/content/articlehtml/2017/ra/c7ra04224h
  5. https://link.springer.com/chapter/10.1007/b13182
  6. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_and_Websites_(Inorganic_Chemistry)/Descriptive_Chemistry/Main_Group_Reactions/Compounds/Hydrides
  7. https://www.wiredchemist.com/chemistry/data/bond_energies_lengths.html
  8. http://staff.ustc.edu.cn/~luo971/2010-91-CRC-BDEs-Tables.pdf
  9. http://hyperphysics.phy-astr.gsu.edu/Nave-html/Faithpathh/wonderwater.html
  10. https://myweb.ecu.edu/bierm/papers/WaterBook.pdf
  11. https://people.bu.edu/mfk/watermodeling.pdf
  12. https://www.usgs.gov/water-science-school/science/specific-heat-capacity-and-water
  13. https://chem.libretexts.org/Bookshelves/General_Chemistry/Chem1_%28Lower%29/07%253A_Solids_and_Liquids/7.03%253A_Hydrogen-Bonding_and_Water
  14. https://www.utdallas.edu/~son051000/chem1312/Chapter16a.pdf
  15. http://www.chem.ucla.edu/14d-s13/Other_Stuff_files/pKa_14D.pdf
  16. https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_%28CK-12%29/23%253A_Electrochemistry/23.09%253A_Electrolysis_of_Water
  17. https://nanoconvergencejournal.springeropen.com/articles/10.1186/s40580-021-00254-x
  18. https://www.monash.edu/student-academic-success/chemistry/reactions-of-organic-compounds/organic-reactions-and-synthesis-pathways/organic-reactions-hydrolysis
  19. https://pmc.ncbi.nlm.nih.gov/articles/PMC9304379/
  20. https://www.usgs.gov/water-science-school/science/how-much-water-there-earth
  21. https://www.usgs.gov/water-science-school/water-cycle
  22. https://wateriso.utah.edu/waterisotopes/pages/information/background.html
  23. https://webbook.nist.gov/cgi/cbook.cgi?ID=C7783064
  24. https://webbook.nist.gov/cgi/cbook.cgi?ID=C7783075
  25. https://webbook.nist.gov/cgi/cbook.cgi?Name=tellurium+hydride
  26. https://www.chem.tamu.edu/rgroup/hughbanks/courses/462/lecturenotes/class6-2.pdf
  27. https://pubchem.ncbi.nlm.nih.gov/compound/Hydrogen-Sulfide
  28. https://wwwn.cdc.gov/TSP/MMG/MMGDetails.aspx?mmgid=385&toxid=67
  29. https://pubchem.ncbi.nlm.nih.gov/compound/Hydrogen-telluride
  30. https://www.researchgate.net/publication/349719907_Bond_Dissociation_Energies_in_Heavy_Element_Chalcogen_and_Halogen_Small_Molecules
  31. https://pubs.acs.org/doi/10.1021/j100271a005
  32. https://www.sciencedirect.com/science/article/abs/pii/S016612800600830X
  33. https://pubchem.ncbi.nlm.nih.gov/compound/Hydrogen-Peroxide
  34. https://pmc.ncbi.nlm.nih.gov/articles/PMC5768111/
  35. https://cdnsciencepub.com/doi/pdf/10.1139/v68-384
  36. https://pubchem.ncbi.nlm.nih.gov/compound/Diselane
  37. https://pubchem.ncbi.nlm.nih.gov/compound/Ditellane
  38. https://www.webqc.org/compound-Te2H2-Te2H2.html
  39. https://uhmreactiondynamics.org/publication_papers/p634.pdf
  40. https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/bph.14330
  41. https://www.thieme-connect.de/products/ebooks/pdf/10.1055/b-0035-108350.pdf
  42. https://repository.geologyscience.ru/bitstream/handle/123456789/21876/Migd_98.pdf?sequence=1
  43. https://acta-arhiv.chem-soc.si/52/52-1-1.pdf
  44. https://www.russchemrev.org/RCR4744pdf
  45. https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/ejic.201100767
  46. https://batadora.trentu.ca/_flysystem/fedora/2022-04/Analysis_and_reactions_of_aqueous_selenide_and_other_reduced_inorganic_selenium_compounds_under_anoxic_conditions.pdf
  47. https://www.sciencedirect.com/science/article/abs/pii/S0277538717302176
  48. https://epdf.pub/inorganic-reactions-and-methods-formation-of-ceramics.html
  49. https://www.pearson.com/channels/general-chemistry/textbook-solutions/mcmurry-8th-edition-9781292336145/ch-15-aqueous-equilibria-acids-bases/aqueous-solutions-of-hydrogen-sulfide-contain-h2s-hs-s2-h3o-oh-and-h2o-in-varyin
  50. https://beckassets.blob.core.windows.net/product/readingsample/105413/9783642039669_excerpt_001.pdf
  51. https://organicchemistrydata.org/hansreich/resources/pka/pka_data/pka-compilation-williams.pdf
  52. https://homework.study.com/explanation/list-the-following-ions-in-order-of-decreasing-basic-strength-ho-hs-hse-hte.html
  53. https://www.sciencedirect.com/science/article/abs/pii/S0045653521032884
  54. https://nvlpubs.nist.gov/nistpubs/Legacy/NSRDS/nbsnsrds46.pdf
  55. https://www.sciencedirect.com/science/article/abs/pii/S1540748902802978
  56. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023GL103195
  57. https://pmc.ncbi.nlm.nih.gov/articles/PMC2396607/
  58. https://pubchem.ncbi.nlm.nih.gov/compound/Deuterium-Oxide
  59. https://gustavus.edu/chemistry/documents/documents/Deuteriumoxide.pdf
  60. https://srd.nist.gov/jpcrdreprint/1.555714.pdf
  61. https://www.mass.gov/doc/january-2020-health-physics-society-tritium-fact-sheet/download
  62. https://www.nature.com/articles/s41598-025-02004-z
  63. https://etd.ohiolink.edu/acprod/odb_etd/ws/send_file/send?accession=akron1522256710854143&disposition=inline
  64. https://inis.iaea.org/records/kdf0r-k9665
  65. https://www.nature.com/articles/s41592-019-0459-y
  66. https://earthobservatory.nasa.gov/features/Paleoclimatology_OxygenBalance
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