[odie5533]

I am having a hard time understanding when a molecule should have a resonance structure.

For SO2, I start by drawing this:
O-S-O
Then I get kind of lost. Oxygen has a higher electronegativity than Sulfur, so it would be more likely to fill its octet (is this even the correct resonaning?):
O=S-O which resonates to O-S=O
But does O=S=O exist? Or how about O (triple bond) S-O?

It seems like for every molecule there are 10 different possible resonance structures, each giving the atoms different formal charges and different number of bonds, but I'm not sure which are the right resonance structures and when to stop making up resonance structures. Any help on this would be greatly appreciated.

[Yggdrasil]

The following tutorial on resonance contributors may be helpful:

http://web.chem.ucla.edu/%7Eharding/tutorials/resonance/imp_res_str.html

If you still have any questions after reading the tutorial, feel free to come back and ask, and we will be glad to help.

[odie5533]

Thanks Yggdrasil, this helped clarify which resonance is important. But I'm still having a bit of trouble with this SO2 problem.

O=S=O seems the most stable. In the page you sent me, it says that the most important factor in determining the most important contributor is to have the most full octets. Of the 3 (O=S=O, O=S-O, O-S=O), only O=S=O has all full octets. Between O=S-O and O-S=O, both are of equal importance. But do they contribute at all to the "real" structure? The question on my quiz asks to draw the lewis dot structure for SO2 and show any important resonance structures. In my professor's answer key he shows O=S-O and O-S=O as the only contributors. Is O=S=O an invalid contributor? Does the real SO2 have 1.5 bonds on each, or the full 2, or is it somewhere between all three?

[0000000]

The first structure on your picture is not stable at all, you know that the most stable form is the one with octet, but count the electrons around the S-atom, there are 10 not 8! That so this structure do not exist.
On the other had the second structure with the resonates have 3 atoms with 8 e on each.

[AWK]

The first structure on your picture is not stable at all, you know that the most stable form is the one with octet, but count the electrons around the S-atom, there are 10 not 8! That so this structure do not exist.
On the other had the second structure with the resonates have 3 atoms with 8 e on each.

Do not be so dogmatic.
try to find octet in eg. PCl5
Octet rule is only abolutely valid for the second period!

[0000000]

But i think that it works like explanation here

[Yggdrasil]

Thanks Yggdrasil, this helped clarify which resonance is important. But I'm still having a bit of trouble with this SO2 problem.

O=S=O seems the most stable. In the page you sent me, it says that the most important factor in determining the most important contributor is to have the most full octets. Of the 3 (O=S=O, O=S-O, O-S=O), only O=S=O has all full octets. Between O=S-O and O-S=O, both are of equal importance. But do they contribute at all to the "real" structure? The question on my quiz asks to draw the lewis dot structure for SO2 and show any important resonance structures. In my professor's answer key he shows O=S-O and O-S=O as the only contributors. Is O=S=O an invalid contributor? Does the real SO2 have 1.5 bonds on each, or the full 2, or is it somewhere between all three?

Because sulfur is a third period element, it can expand its valence shell to include d-orbitals and have more than eight electrons in its valence shell.  So, I would expect sulfur to have two double bonds.  However, most of the reliable sources I can find say that sulfur has the resonance structure that you've shown.  I don't really know why this is, unfortunately.

[odie5533]

What sources do you look to for resonance structure listing?

Even if a resonance form is unstable (even highly unstable), when asked to list resonance forms, should ALL possible forms be listed?

My actual quiz question was to show the resonance for HONO. Here is my answer:
[ H-O-N=O   H-O=N-O   H-O-N-O ]
Formal charges and lone pairs not shown. Of them, the most stable is H-O-N=O, but the others are valid resonance forms, right?

[MitchTwitchita]

The first two definitely look good.  You might have a problem with the valence electrons on the Nitrogen in the last one though.

[odie5533]

H-O-N-O
2 pairs on the first O, 1 pair on the N, and 3 pairs on the last O. N with a +1 charge and the last O with a -1 charge.

[Yggdrasil]

Yes, they are all valid resonance structures.  As you noted, H-O-N=O is the most stable, so the real molecule will most closely resemble that structure, but the other resonance structures would make some contribution to the structure (although as MitchTwitchita mentioned, the thrid resonance structure is not going to be an important contributor because of the unfilled octet of the nitrogen).  It may be best to ask your teacher on his/her preference on which resonance structures to show.

As for the sources for SO₂, these two pages both say that SO₂ has two resonance structures and don't mention the O=S=O form.

http://www.cem.msu.edu/~reusch/VirtualText/intro3.htm
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/lewis.html#hybrid

In addition, bond lengths derived from the following paper seem to indicate S-O bond lengths consistent with 1.5 bonds between the S and O instead of 2 bonds between the S and O.  (Although the paper is from 1935, so maybe something has changed since then).

Cross, P.C. & Brockway, L.O. The Molecular Structures of Sulfur Dioxide, Carbon Disulfide, and Carbonyl Sulfide. The Journal of Chemical Physics 3, 821-824 (1935).  DOI:10.1063/1.1749599

[odie5533]

This is the only page I found that supports me: http://www.uwosh.edu/faculty_staff/gutow/Lewis_Tutorial/SO2_8.html

Are resonance structures determinable? Or is it all just guessing? And if it is all guessing, do resonance structures even matter? I mean, on a big molecule, I could sit for hours making up resonance structures, never knowing if any of them even occur. And even if I do "guess" right, I'd still need to confirm my guess somehow, which means I just wasted my time guessing since I'd then have the correct form anyway.

Bah, I think I'm rambling too much... I've been studying chemistry for over a year and still nothing makes sense

[Yggdrasil]

Resonance structures are determinable in that they allow us to make and test specific predictions about a molecule's structure and properties.  For example, in an amide, you can draw a resonance structure where the double bond occurs between the oxygen and nitrogen instead of the oxygen and carbon.  From this resonance structure, we can infer that rotation about the C-N bond will not occur because of the partial double-bond character of the bond.  This has been shown to be true.

As for resonance structures, its important to remember that resonance structures don't actually exist.  The structure of a molecule will be the superposition of the molecule's resonance structures (the resonance hybrid).

The concept of resonance structures is helpful for introducing the concept of conjugated pi system.  However, in more advanced chemistry, I believe it is more useful to think of conjugated pi systems instead of resonance structures.

Anyway, don't feel too bad about not being able to fully grasp everything in chemistry.  Chemistry takes a long time to learn.  I've been studying chemistry for much longer than you and sometimes it seems like certain basic concepts still don't make sense to me at all   But, that's what makes chemistry fun.  There's always more cool stuff you can learn.

[AWK]

There is no need to draw resonance structures for HON=O. All others structures should have splitted charges. For this above an octet rule is fulfilled.