Draw a single lewis structure for the phosphate ion (po43−), assuming that the phosphorus atom can expand its octet such that the formal charge on each of the atoms is minimized. include all formal charges and lone-pair electrons.

The Lewis diagram with single bonds, and all the oxygens have 3 lone pairs.  PO4^3- has 5+4x6+3 = 32 e-

.. O 

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O-P-O 

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...O 

The only purpose that you might want to contain a double bond is for the reason that of an excessively strict adherence to formal charges. By creating one of the bonds a double bond you decrease the formal charge on P from +2 to +1 and one of the oxygens from -1 to 0. 

The detail is that all four bonds are undistinguishable. If one of the bonds was a double bond then one bond would have a superior energy that the others. That is why you could use resonance, to make all the bonds equal.

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