Structural Formula of Polyatomic ions

[utopiapraxis]

Hello, I'm very new to chemistry so bear with me if I seem to be too much of a newbie..hehe.

I seem to be having trouble understanding the structural formulas of polyatomic ions. Can somebody please explain to me how to draw the structural formula of a phosphate ion, PO4^3-. <-----(hope that makes sense; please correct me if it doesn't). I have attempted this and my solution is the phosphorus atom bonded to four oxygen atoms each with a single covalent bond. The answer I have retrieved off of various online databases however, suggests the phosphorus atom bonded to four oxygen atoms, three with a single covalent bond, the fourth O atom with a double covalent bond. Which one is right? Please help!! I get the impression that my error has something to do with the fact that one or more of the oxygen atoms are bonded as a coordinate covalent bond in which case I still have no clue of what's going on.

Thanks in advance,
Kevin