Oxyanion

Key takeaways

Oxyanion belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Oxyanion to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Oxyanion from memory before moving on to harder problems.

Reference excerpt

An oxyanion, or oxoanion, is an ion with the generic formula AxOz−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 HwAxOz+y. The structures of condensed oxyanions can be rationalized in terms of AOm 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.

Monomeric oxyanions The formula of monomeric oxyanions, AOn−m, 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 an electrical charge that is too high and undergo hydrolysis. 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 H 3 I O 6 2 − ⇌ H 2 I O 6 3 − + H + {\displaystyle \mathrm {\ H_{3}IO_{6}^{2-}\rightleftharpoons \ H_{2}IO_{6}^{3-}\ +\ H^{+}\ } } having pKa = 11.60 .

Naming 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.

If central atom is not in Group VII or VIII

If central atom is in Group VII or VIII

Condensation reactions

In aqueous solution, oxyanions with high charge can undergo condensation reactions, such as in the formation of the dichromate ion, Cr2O2−7:

2 CrO 4 2 − + 2 H + ↽ − − ⇀ Cr 2 O 7 2 − + H 2 O {\displaystyle {\ce {2 CrO4^2- + 2 H+ <=> Cr2O7^2- + H2O}}}

The driving force for this reaction is the reduction of electrical charge density on the anion and the elimination of the hydronium (H3O+) 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.

The conversion of ATP to ADP is a hydrolysis reaction and is an important source of energy in biological systems. The formation of most silicate minerals can be viewed as the result of a de-condensation reaction in which silica reacts with a basic oxide, an acid–base reaction in the Lux–Flood sense.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Oxyanion

Start with the simplest possible case. Write down what Oxyanion claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Oxyanion before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Oxyanion ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Oxyanion

In research: Oxyanion appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Oxyanion in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Oxyanion is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acid–base chemistry, Equilibrium chemistry, Oxyanions, so understanding it makes those chapters shorter.
In everyday life: Look for Oxyanion outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Oxyanion in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Oxyanion means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Oxyanion out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Oxyanion in simple terms?

An oxyanion, or oxoanion, is an ion with the generic formula AxOz−y (where 'A' represents a chemical element and 'O' represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements.

Why does Oxyanion matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Oxyanion?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Oxyanion.