Question

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

Answer 1

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.

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.