Dumb Question Please Help (GChem)

[NumbaOneStunna]

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I feel real dumb, this was on the ADA practice test. NO3- resonance structures. How many and what are they? (answer is 3) I see six potential:

3 of them (With A N=O and the double bond on diff O each time.)
But why three, aren't these all the same thing if you rotate the molecule? Or should I not think of that when doing resonance structures since the idea is that the double bond is distributed among three O's?


3 of them (With all N-O and no double bond)
Why does this form not count? I know the Nitrogen won't have a full octet and this makes for a crappy resonance form - but it's still a resonance form - right? What is the general rule that says whether or not a resonance form counts.


*confused* and two days before my DAT.

 

[goffdent]

don't rotate the molecule when doing figuring resonance. it's not considered the same molecule even though that's what you would definitely think.

3x N-O doesn't work because the formal charges won't add up to a negative 1

 

[goffdent]

remember that formal charge has a large impact on the stability. The more electronegative atom should get the negative formal charge, Oxygen in this case.

 

[NumbaOneStunna]

Are you sure the formal charge changes? The N is (5-3)= +2 and each O is -1 for a -1 total. (So for a phosphate ionic ligand the -3 represents the sum of the formal charges right? surprisingly I didn't know that, thanks)

I think in both cases oxygen gets the negative formal charge. I get that the reason why the case with single N without a lone pair bonded to three oxygens is b/c its octet is not full. I dont get why you wouldnt count this form though. Is it b/c of the octet not being full? Anyone have a good answer to this? Thanks!

 

[NumbaOneStunna]

Bump

 

[jorasic]

Dude that question pissed me off too.
Forget that and move on. You KNOW how to do all that.
The DAT won't trick you or make it oblivious like that.
GL!

 

[NumbaOneStunna]

But it was the practice DAT! I'd go crazy if i got a resonance question and don't know what the DAT is looking for.