How many resonance structures can be drawn for #"PO"_4^(3-)#?

1 Answer
Ernest Z.
Jan 12, 2016

You can draw five resonance structures for #"PO"_4^(3-)#, but one of them is a minor contributor to the resonance hybrid.

Explanation:

If you start by drawing four oxygen atoms single bonded to a phosphorus atom and give every atom an octet, you get Structure R in the diagram below.

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This is not a "good" structure , because it has five atoms with formal charges.

We can reduce the number of charges by moving electrons to convert a #"P-O"# single bond to a #"P=O"# double bond.

There are four #"P-O"# bonds, so we can draw four structures: S, T, U, and V.

These are all equivalent and are the major contributors to the resonance hybrid.

Hence, the actual structure is a resonance hybrid of all five structures, with : S, T, U, and V being the major contributor and R a minor contributor.

chemwiki.ucdavis.edu

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