Why doesn't the formula of chlorine nitrate contain a positive charge?

[zenterix]

Homework Statement

Write the Lewis structure of the species chlorine nitrate, $ClONO_2$.

Relevant Equations

At this point, this problem incorporates material on simple Lewis structures and exceptions to the octet rule.

Looking up on the internet the structure is

My question is why (or, better, how) do we know that there is that positive charge on the nitrogen?

Wouldn't the overall molecule have a positive charge in it? Then why is this not denoted in the formula $ClONO_2$?

 

[TeethWhitener]

A +1 charge and a -1 charge on the same molecule mean that the overall molecule is neutral.

 

[zenterix]

True I hadn't even noticed the negative charge on the oxygen there. However, at this point (and I know that the theory I am learning currently is very limited) such structures seem very ad hoc.

At this point in the theory I would have predicted that the chlorine would have expanded valence, forming two double bonds with oxygens and a triple bond with the nitrogen.

 

[TeethWhitener]

Writing it as $ClONO_2$ instead of $ClNO_3$ gives a hint about its structure.

Also, think about the maximum oxidation state that chlorine can have and what it would mean if it had the bonds you described in post 3.

 

[Borek]

Note: Lewis structures are just a simplified approximation. They do give some interesting insight into what we can expect and whether the molecule can be stable, but they shouldn't be treated religiously seriously.

 

[DrJohn]

If you look at the wikipedia entry for this compound, what do you notice about the N-O bond lengths? What does that tell you about the true nature of the bonding and the presence or absence of electric charges?

 

[zenterix]

Writing it as $ClONO_2$ instead of $ClNO_3$ gives a hint about its structure.

Also, think about the maximum oxidation state that chlorine can have and what it would mean if it had the bonds you described in post 3.

I made some confusion initially because I thought there were two oxygens not three. With three oxygens, there aren't enough valence electrons in chlorine to form double bonds with three oxygens and a triple bond with a nitrogen.

I'm not sure what the maximum oxidation state of chlorine is, I'm guessing +7 since if chlorine were to form seven perfectly ionic bonds and lose an electron in each it would have +7 oxidation state.

However, chlorine is more electronegative than nitrogen, so minus six electrons (lost to the oxygens) plus 3 electrons from the nitrogen would +3 oxidation state.

Are you saying that if chlorine bonds with all the atoms in the molecule the oxidation state would exceed the maximum?

 

[TeethWhitener]

Yes that was my idea, but you’re right, it doesn’t quite work out that way with electronegativity and oxidation state.

The more proper way to evaluate it is probably by formal charge. See if you can work out the formal charges with your proposed structures.

Honestly, for me it was more just a gut intuition that 9 bonds around chlorine is completely impossible.

 

[zenterix]

If you look at the wikipedia entry for this compound, what do you notice about the N-O bond lengths? What does that tell you about the true nature of the bonding and the presence or absence of electric charges?

There are three N-O bonds, two have length 119.6pm and one has length 149.9pm.

The two with length 119.6pm correspond to a double bond and a bond between a positively charged N and a negatively charged O.

Multiple bonds are shorter than single bonds between the same two elements. The fact that our double bond has the same length as the single bond indicates that something is making the single bond stronger than usual.

This something I guess is this polarization of the bond. It is still not clear to me how it comes about, however.

Nitrogen has five valence electrons, and is sharing three with two oxygens. Then, apparently, the bond with the last oxygen happens with both electrons being from nitrogen (is this a coordinate covalent bond?).

So the nitrogen is sharing both electrons with the last oxygen, and so we characterize this as the nitrogen having "lost" one of them to the oxygen and thus having a positive charge.

Is this how it works?

 

[DrJohn]

The two N-O bonds that are the shorter pair are not really double bonds. Think how the N can share three electrons with two O and yet they have the same length. Have you thought about benzene, its structure and the bond lengths?

And note what Borek said.