# Determining the number of energy equivalent resonance structures

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in H2AsO4-.

I figured it was four, but I am mistaken. Any pointers?

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in H2AsO4-.

I figured it was four, but I am mistaken. Any pointers?

There are only two resonance structures that don't involve an oxygen atom taking a +1 formal charge. Do electronegative elements enjoy being electron deficient?

Edited by mississippichem
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There are only two resonance structures that don't involve an oxygen atom taking a +1 formal charge. Do electronegative elements enjoy being electron deficient?

My idea of resonance structures must be Wrong

As is the central atom with two (-O-H), one =O and one -O(-) off it.

That is the Lewis structure. Couldn't the (-) charge be on any oxygen, making a minimum of 4 resonance structures?

Edited by CrazCo
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My idea of resonance structures must be Wrong

As is the central atom with two (-O-H), one =O and one -O(-) off it.

That is the Lewis structure. Couldn't the (-) charge be on any oxygen, making a minimum of 4 resonance structures?

-Two electrons can be moved from the As=O bond to the As-O bond. The single and double bond effectively "switch out". If those $\pi$ electrons were moved to an As-O bond where the oxygen is linked to a hydrogen (As-O-H), then oxygen would be stuck with three bonds. There are actually many resonance structures but most of them are of significantly higher energy and make minimal contributions to the resonance hybrid.

-Moving any of the single bonds would disconnect atoms from the molecule which violates the definition of a resonance structure. Breaking bonds and allowing charge separation falls under the umbrella of Dewar Structures.

-Remember oxygen is a very electronegative element, and would usually prefer a negative formal charge or oxidation state.

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