naming compounds with three elements again?

Question

so thanx to those who answered my previous question which was specifically about phosphoric acid (H3PO4)

someone said that the reason why H became 3 because you need three um, was that charge? whatever, you need 3 thingies to balance it with PO4

but how did you come up with PO4? is there a way to get the 4? coz P's charge is 3 and O's charge is 2 and i dont get how 4 came out.

help again pleeeeese!

and thanx!!

Answer 1

PO4(3-) is the correct formula. The formal charge per oxygen is -2, making a total of -8. The formal charge of P here is +5 (not 3). Add them and you get -3.