What is the Lewis dot structure for PO4 3-?

Question

I'm confused because when I draw all of the Os attached with single bonds to P the formal charges don't make sense. But when I add a double bond to one of the Os the formal charges are -1 to 3 of the oxygens. I'm confused if there should be a double bond or not for the PO4 3- structure.

Answer 1

If we start at the beginning by drawing the Lewis dot structure we start with valence electrons.

P has 5 ve

each O has 6 ve so that's a total of 4x6 = 24 ve

We add 3 more for the 3- charge for a total of 32 valence electrons

So, this can be written with P at the center and 4 O atoms attached with single bonds and all O atoms would have 3 lone pairs. This arrangement gives a formal charge of +1 on P and -1 on all O atoms. Not good.

Adding a double bond to one of the O atoms results in a formal charge of zero on P, zero on the O with the double bond, and -1 on the other 3 O atoms. This is fine because the charge on the phosphate anion is 3- and so the -1 on each of 3 oxygens accounts for this. You shouldn't be confused because you have done it correctly. Also recall that P can have an expanded octet (which it does in the dot structure we just drew).