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Oxyanion
An oxyanion, or oxoanion, is an ion with the generic formula A
xOz−
y (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The corresponding oxyacid of an oxyanion is the compound H
zA
xO
y. The structures of condensed oxyanions can be rationalized in terms of AOn polyhedral units with sharing of corners or edges between polyhedra. The oxyanions (specifically, phosphate and polyphosphate esters) adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are important in biology.
The formula of monomeric oxyanions, AOm−
n, is dictated by the oxidation state of the element A and its position in the periodic table. Elements of the first row are limited to a maximum coordination number of 4. However, none of the first row elements has a monomeric oxyanion with that coordination number. Instead, carbonate (CO2−
3) and nitrate (NO−
3) have a trigonal planar structure with π bonding between the central atom and the oxygen atoms. This π bonding is favoured by the similarity in size of the central atom and oxygen.
The oxyanions of second-row elements in the group oxidation state are tetrahedral. Tetrahedral SiO4 units are found in olivine minerals, (Mg,Fe)2SiO4, but the anion does not have a separate existence as the oxygen atoms are surrounded tetrahedrally by cations in the solid state. Phosphate (PO3−
4), sulfate (SO2−
4), and perchlorate (ClO−
4) ions can be found as such in various salts. Many oxyanions of elements in lower oxidation state obey the octet rule and this can be used to rationalize the formulae adopted. For example, chlorine(V) has two valence electrons so it can accommodate three electron pairs from bonds with oxide ions. The charge on the ion is +5 − 3 × 2 = −1, and so the formula is ClO−
3. The structure of the ion is predicted by VSEPR theory to be pyramidal, with three bonding electron pairs and one lone pair. In a similar way,
The oxyanion of chlorine(III) has the formula ClO−
2, and is bent with two lone pairs and two bonding pairs.
In the third and subsequent rows of the periodic table, 6-coordination is possible, but isolated octahedral oxyanions are not known because they would carry too high an electrical charge. Thus molybdenum(VI) does not form MoO6−
6, but forms the tetrahedral molybdate anion, MoO2−
4. MoO6 units are found in condensed molybdates. Fully protonated oxyanions with an octahedral structure are found in such species as Sn(OH)2−
6 and Sb(OH)−
6. In addition, orthoperiodate can be only partially deprotonated, with
The naming of monomeric oxyanions follows the following rules.
Here the halogen group (group 7A, 17) is referred to as group VII and the noble gases group (group 8A) is referred to as group VIII.
In aqueous solution, oxyanions with high charge can undergo condensation reactions, such as in the formation of the dichromate ion, Cr2O2−7:
The driving force for this reaction is the reduction of electrical charge density on the anion and the elimination of the hydronium (H+) ion. The amount of order in the solution is decreased, releasing a certain amount of entropy which makes the Gibbs free energy more negative and favors the forward reaction. It is an example of an acid–base reaction with the monomeric oxyanion acting as a base and the condensed oxyanion acting as its conjugate acid. The reverse reaction is a hydrolysis reaction, as a water molecule, acting as a base, is split. Further condensation may occur, particularly with anions of higher charge, as occurs with adenosine phosphates.
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Oxyanion
An oxyanion, or oxoanion, is an ion with the generic formula A
xOz−
y (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The corresponding oxyacid of an oxyanion is the compound H
zA
xO
y. The structures of condensed oxyanions can be rationalized in terms of AOn polyhedral units with sharing of corners or edges between polyhedra. The oxyanions (specifically, phosphate and polyphosphate esters) adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are important in biology.
The formula of monomeric oxyanions, AOm−
n, is dictated by the oxidation state of the element A and its position in the periodic table. Elements of the first row are limited to a maximum coordination number of 4. However, none of the first row elements has a monomeric oxyanion with that coordination number. Instead, carbonate (CO2−
3) and nitrate (NO−
3) have a trigonal planar structure with π bonding between the central atom and the oxygen atoms. This π bonding is favoured by the similarity in size of the central atom and oxygen.
The oxyanions of second-row elements in the group oxidation state are tetrahedral. Tetrahedral SiO4 units are found in olivine minerals, (Mg,Fe)2SiO4, but the anion does not have a separate existence as the oxygen atoms are surrounded tetrahedrally by cations in the solid state. Phosphate (PO3−
4), sulfate (SO2−
4), and perchlorate (ClO−
4) ions can be found as such in various salts. Many oxyanions of elements in lower oxidation state obey the octet rule and this can be used to rationalize the formulae adopted. For example, chlorine(V) has two valence electrons so it can accommodate three electron pairs from bonds with oxide ions. The charge on the ion is +5 − 3 × 2 = −1, and so the formula is ClO−
3. The structure of the ion is predicted by VSEPR theory to be pyramidal, with three bonding electron pairs and one lone pair. In a similar way,
The oxyanion of chlorine(III) has the formula ClO−
2, and is bent with two lone pairs and two bonding pairs.
In the third and subsequent rows of the periodic table, 6-coordination is possible, but isolated octahedral oxyanions are not known because they would carry too high an electrical charge. Thus molybdenum(VI) does not form MoO6−
6, but forms the tetrahedral molybdate anion, MoO2−
4. MoO6 units are found in condensed molybdates. Fully protonated oxyanions with an octahedral structure are found in such species as Sn(OH)2−
6 and Sb(OH)−
6. In addition, orthoperiodate can be only partially deprotonated, with
The naming of monomeric oxyanions follows the following rules.
Here the halogen group (group 7A, 17) is referred to as group VII and the noble gases group (group 8A) is referred to as group VIII.
In aqueous solution, oxyanions with high charge can undergo condensation reactions, such as in the formation of the dichromate ion, Cr2O2−7:
The driving force for this reaction is the reduction of electrical charge density on the anion and the elimination of the hydronium (H+) ion. The amount of order in the solution is decreased, releasing a certain amount of entropy which makes the Gibbs free energy more negative and favors the forward reaction. It is an example of an acid–base reaction with the monomeric oxyanion acting as a base and the condensed oxyanion acting as its conjugate acid. The reverse reaction is a hydrolysis reaction, as a water molecule, acting as a base, is split. Further condensation may occur, particularly with anions of higher charge, as occurs with adenosine phosphates.