# Homework Help: Resonance structures

1. Mar 17, 2007

### oceanflavored

1. The problem statement, all variables and given/known data
i am studying for my apchem exam in may.and got stuckup on a question. it's just a little exercise.so the answers aren't there.but ok here it is;
give the resonance structures of PO4 (3-) and there's supposed to be four

2. Relevant equations
none?

3. The attempt at a solution
i started drawing it so..
PO4 (3-) = 5 + (6*4) +3 = 32/2 = 16 pairs of electrons so...

O
|
O -- P --- O
|
O

so there's the single bonds (8 electrons: octet rule) for P and then 6 electrons around each O to complete O's octet rule.
and my problem is...where's the dang resonance structure? because everyone gets their electrons.and is happy. unless of course P has an expanded octet and gets more than 8?
'ppreciate it alottttt.

2. Mar 17, 2007

### oceanflavored

sorry.

ewwww.my molecular structure messed up. hopefully you guys will understand what i'm talking aboutt.
<- i like that smiley

3. Mar 17, 2007

### tim_lou

how about one double bound, two single bound and another oxygen atom just dangling around, sucking the electron from the P and not sharing any?

I might be wrong though... it's been a while since that ap chem exam....

4. Mar 18, 2007

### t!m

This isn't a resonance structure - by definition, connectivity must remain unchanged; you can only move electrons.

So you can take a lone pair of electrons from an oxygen and make another bond (thus creating a P=O) and leave the others unchanged - this can be done for each oxygen, creating four different resonance structures. Although this gives P ten valence electrons, it results in a formal charge of 0 for both P and O.

5. Mar 18, 2007

### Moonbear

Staff Emeritus
Just a reminder to those answering the question...try not to just offer answers (especially ones you're uncertain of) and help guide those asking the questions to find their own answers.

Oceanflavored, let's start out with the basics so we can assess where you're getting stuck.

So, first explain to us in your words what a resonance structure is (yes, we know, I want to know if you know).

Second, when you write PO4 (3-), what does the 3- signify?

Third, you put a note that your Lewis structure is "messed up," and indeed it is, but in order for us to know what you do and don't know, we need to know what part of the Lewis structure YOU think is messed up. So since you haven't been able to type it out, can you describe to us in words what you are starting with? Start with the P and then describe where the 4 oxygens are relative to that and how many bonds you are assigning for each position. For the sake of clarity, you can use terms such as up, down, left, right, or north, south, east, west...whatever helps tells us what direction you are drawing bonds sticking out of each atom in your diagram.

Lastly, as you draw the Lewis structure, remember to put the dots on it for where the electrons are.

We will wait for your next response before continuing forward, because I suspect I see where you're running into difficulty, but need to confirm it and figure out why you're stuck there before helping you figure this one out.