In: Chemistry
Draw a Lewis structure for PO4^3- that minimizes formal charges.
Well, actually, you can draw the Lewis diagram with single bonds, and all the oxygens have 3 lone pairs.
PO4^3- has 5+4x6+3 = 32 e-
The only reason that you might want to include a double bond is because of overly strict adherence to formal charges. By making one of the bonds a double bond, you reduce the formal charge on P from +2 to +1 and one of the oxygens from -1 to 0.
The fact is that all four bonds are identical. If one of the bonds were a double bond, then one bond would have greater energy than the others. That is why you could use resonance to make all the bonds equivalent.