[headfrozen]

I was asked as homework question to draw the Lewis structure for PO₄^3-_(aq).
I drew something that looked like this:

and this is a picture from the answer sheet we were given so I thought it was correct.

But when i looked it up I got this:

which is what the teacher drew when we were taking it up. So which one is correct ? Both satisfy the electron budget but I don't understand why the second one is somehow better.

[chenbeier]

Yours would be PO4 4-, what is not existing. Phosphorous would have 1 not bounded Electron left.

[AWK]

Both structures are correct, the second one is preferred.

[headfrozen]

Yours would be PO4 4-, what is not existing. Phosphorous would have 1 not bounded Electron left.

Would that mean the negative charges come from 3 of the oxygens being O^- ions, and that 3/5 of the bonds are coordinate covalent bonds?

[AWK]

In the case of the first structure, we have a +1 charge on phosphorus and four negative charges on oxygen atoms, and the phosphorus atom strictly follows the octet rule. But the phosphorus atom may have an octet extended, as in the second formula.

[chenbeier]

On the drawing the phosphorous wasnt marked with a positiv charge. So the molecules is not existing. If there is one charge then iyou are correct.

[AWK]

Lewis structures show electrons. Formal charges can be determined very easily from such structures.