Interchalcogen

The chalcogens react with each other to form interchalcogen compounds.[1]

Although no chalcogen is extremely electropositive,[note 1] nor quite as electronegative as the halogen fluorine (the most electronegative element), there is a large difference in electronegativity between the top (oxygen = 3.44 — the second most electronegative element after fluorine) and bottom (polonium = 2.0) of the group. Combined with the fact that there is a significant trend towards increasing metallic behaviour while descending the group (oxygen is a gaseous nonmetal, while polonium is a silvery post-transition metal[note 2]), this causes the interchalcogens to display many different kinds of bonding: covalent, ionic, metallic, and semimetallic.[note 3][1]

Known binary interchalcogens

	O
O	O₂, O₃, O₄, O₈	S
S	S₂O, SO, S₂O₂, SO₂, SO₃	S₂, S₃, S₆, S₇, S₈, S_∞	Se
Se	SeO₂, SeO₃	Se_xS_y	Se₆, Se₇, Se₈, Se_∞	Te
Te	TeO, TeO₂, TeO₃, Te₂O₅	Te_xS_y (many unknown)	Te_xSe_y (many unknown)	Te_∞	Po
Po	PoO, PoO₂, PoO₃	PoS (many unknown)	Po_xSe_y (many unknown)	Po_xTe_y (unknown)	Po_∞

Bonding in the binary interchalcogens

Going down the above table, there is a transition from covalent bonding (with discrete molecules) to ionic bonding; going across the table, there is a transition from ionic bonding to metallic bonding. (Covalent bonding occurs when both elements have similar high electronegativities; ionic bonding occurs when the two elements have very different electronegativities, one low and the other high; metallic bonding occurs when both elements have similar low electronegativities.) For example, in the leftmost column of the table (with bonds to oxygen), O₂ and O₃ are purely covalent, SO₂ and SO₃ are polar molecules, SeO₂ forms chained polymers (stretching in one dimension), TeO₂ forms layered polymers (stretching in two dimensions), and PoO₂ is ionic with the fluorite structure (spatial polymers, stretching in three dimensions); in the bottom row of the table (with bonds to polonium), PoO₂ and PoS are ionic, Po_xSe_y and Po_xTe_y are semimetallic, and Po_∞ is metallic.[1]

Summary of known binary interchalcogens

Sulfur chalcogenides

Molecular structure of sulfur monoxide.

Lower sulfur oxides, S_xO_y where the ratio X:Y is greater than 1:2
- Disulfur monoxide, S₂O
- Disulfur dioxide, S₂O₂
- Sulfur monoxide, SO
Sulfur dioxide, SO₂
Sulfur trioxide, SO₃
Higher sulfur oxides, SO_x where x>3

Selenium chalcogenides

Molecular structure of selenium trioxide.

Selenium dioxide, SeO₂
Selenium trioxide, SeO₃
Many "alloys" of selenium and sulfur in different concentrations with semimetallic bonding, Se_xS_y
- "Selenium monosulfide", SeS
- "Selenium disulfide", SeS₂, actually a 2:1 mixture of cyclo-Se₃S₅ and cyclo-Se₂S₆
- "Selenium trisulfide", SeS₃, actually occurring as the cyclic dimer Se₂S₆

Tellurium chalcogenides

Crystal structure of tellurium dioxide.

Tellurium monoxide, TeO (unstable transient species)
Tellurium dioxide, TeO₂
Tellurium trioxide, TeO₃
Ditellurium pentoxide, Te₂O₅[2]
Many "alloys" of tellurium and sulfur in different concentrations with semimetallic bonding, Te_xS_y
Many "alloys" of tellurium and selenium in different concentrations with semimetallic bonding, Te_xSe_y

Polonium chalcogenides

Unit cell of polonium dioxide (cubic modification). Po: white; O: yellow.

Polonium monoxide, PoO
Polonium dioxide, PoO₂
Polonium trioxide, PoO₃
Polonium monosulfide, PoS
Many "alloys" of polonium and selenium in different concentrations with semimetallic bonding, Po_xSe_y
Many "alloys" of polonium and tellurium in different concentrations with semimetallic bonding, Po_xTe_y

Notes

This article uses Pauling electronegativity throughout.
The classification of polonium as a post-transition metal or a metalloid is disputed.
The heavier halogens are sufficiently electronegative to prevent ionic or metallic bonding in the interhalogens, and the lighter pnictogens are not sufficiently electronegative to allow ionic or metallic bonding in the interpnictogens.

References

Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, pp. 585–586, ISBN 0-12-352651-5
Lindqvist, O.; Moret, J. (1973). "The crystal structure of ditellurium pentoxide, Te2O5". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry. 29 (4): 643–650. doi:10.1107/S0567740873003092.

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[2] This article uses Pauling electronegativity throughout.

[Po-3] The classification of polonium as a post-transition metal or a metalloid is disputed.

[4] The heavier halogens are sufficiently electronegative to prevent ionic or metallic bonding in the interhalogens, and the lighter pnictogens are not sufficiently electronegative to allow ionic or metallic bonding in the interpnictogens.

[Holleman&Wiberg-1] Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, pp. 585–586, ISBN 0-12-352651-5

[5] Lindqvist, O.; Moret, J. (1973). "The crystal structure of ditellurium pentoxide, Te2O5". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry. 29 (4): 643–650. doi:10.1107/S0567740873003092.