3 resonance structures for NO3- ? on 2007 ADA exam

[Member 8586]

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how are there 3 resonance structures for NO3- ? the double bond moving onto different O's? idk..maybe i wasnt thinking straight but i thought they would all be the same if the double bond shifts to different O's because it essentially all made the same thing - a double bonded O in the middle with 2 single bonded Os around it

 

[premac]

what are the 3 resonance structures of NO3- ....I only came up with two...i figured a single bond connecting every N-O and 10 lone pairs total....or 1 double bond and 2 single bonds connecting the N-O's with 9 lone pairs ....what else?

You can't use 3 single bonds, because that won't fill the octet of the nitrogen. And nitrogen isn't one of those "exceptions" that can defy the law.
The 3 resonance structures would all contain 1 double bond and 2 single bonds with 8 lone pairs.
Resonance can be stabilized by two other oxygen atoms, so that's why there are 3 resonance stuctures, total.
I think the problem you had was that you were assuming double bonded oxygen on one end was the same as double bonded oxygen on the other end. This would mean they're "identical" structures in terms of organic chemistry, but when you're looking at resonance, you're looking at how the electrons can be redistributed throughout.
Hope that helps!

 

[Member 8586]

You can't use 3 single bonds, because that won't fill the octet of the nitrogen. And nitrogen isn't one of those "exceptions" that can defy the law.
The 3 resonance structures would all contain 1 double bond and 2 single bonds with 8 lone pairs.
Resonance can be stabilized by two other oxygen atoms, so that's why there are 3 resonance stuctures, total.
I think the problem you had was that you were assuming double bonded oxygen on one end was the same as double bonded oxygen on the other end. This would mean they're "identical" structures in terms of organic chemistry, but when you're looking at resonance, you're looking at how the electrons can be redistributed throughout.
Hope that helps!

originally i was counting 26 electrons for some reason lol so that was my first mistake - hence the edited post but i still didnt get it because yea - like u said i thought they were all identical...hmm good to know...thanks a lot

 

[Member 8586]

You can't use 3 single bonds, because that won't fill the octet of the nitrogen. And nitrogen isn't one of those "exceptions" that can defy the law.
The 3 resonance structures would all contain 1 double bond and 2 single bonds with 8 lone pairs.
Resonance can be stabilized by two other oxygen atoms, so that's why there are 3 resonance stuctures, total.
I think the problem you had was that you were assuming double bonded oxygen on one end was the same as double bonded oxygen on the other end. This would mean they're "identical" structures in terms of organic chemistry, but when you're looking at resonance, you're looking at how the electrons can be redistributed throughout.
Hope that helps!

originally i was counting 26 electrons for some reason lol so that was my first mistake - hence the edited post but i still didnt get it because yea - like u said i thought they were all identical...hmm good to know...thanks a lot

wait so actually though... the double bond cant just come off and then leave an oxygen with an extra lone pair and negative charge... and the N with a lone pair on it (kinda like NH3)...? im clearly not a big fan of resonance lol

EDIT nvm .. lone pair on N would make a total of 26 electrons then... and without the lone pair it wouldnt have an octet.. ok.. blah