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After consulting three organic chemistry texts (Wade, Klein, and Brown), only one actually clearly delineates a few guidelines for picking the most significant resonance structure. The others mention the same factors (maybe less one or two). So in sum, the important factors to consider are:

1) A filled valance shell is preferable to a partially filled valance shell.

2) The more covalent bonds the better.

3) Least separation of oppositely signed charges. I'm thinking this is a Coulombic effect. Some books imply formal charge through the term "charge" but what about partial charges (which are much more reflective of true charges than formal charges. Which one should I really be looking at? Least separation of opposite formal charges or opposite partial charges?

4) Negative charges should generally be assigned to the more electronegative elements; the opposite is true for electropositive elements. Again, formal charge misleads - consider the ammonium ion (positive formal charge on nitrogen; negative partial charge on nitrogen). Again, one book implies formal charge but really, isn't looking at partial charges a bit more informative?

5) Unmentioned factor in any of the three textbooks: What about hybridization? If we have a molecule with two nitrogens, one $\ce{sp}$ hybridized and one $\ce{sp^2}$ hybridized but otherwise identical, shouldn't we also take into account "orbital electronegativity" - i.e. the ability of hybrid orbitals with higher s-character to better stabilize negative charge (and conversely higher p-character better stabilizes positive charge)?

6) Unmentioned factor in any of the three textbooks: Induction? Shouldn't induction also play a role in resonance stabilization? Resonance delocalizes electron(s); can't induction further stabilize electrons? debunked.

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  • $\begingroup$ Inductive effects can weaken resonance structures by localizing the electrons (observe the effect of various substituents on aromatic rings). You can't have resonance on $sp^3$ hybridized molecules $\endgroup$
    – Dan Oberlam
    May 18 '14 at 23:19
  • $\begingroup$ Good points. I'll edit the text to reflect your catch! I should probably have written sp2 hybridization or something! And I should have caught that part about inductive effects localizing reaction; I just had a question answered earlier today about enols. I learned that the hydroxyl group of enols concentrates electron density in the C=C double bond adjacent to the alcohol group. $\endgroup$
    – Dissenter
    May 18 '14 at 23:23
  • $\begingroup$ Both ammonia and ammonium ion have nitrogen with a formal negative charge (to balance the positive hydrogens). $\endgroup$
    – LDC3
    May 19 '14 at 0:35
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    $\begingroup$ Thank you for the comment! Another good term to research! $\endgroup$
    – Dissenter
    May 19 '14 at 4:08
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    $\begingroup$ What is the question here? $\endgroup$
    – Lighthart
    May 19 '14 at 20:34
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As far as hybridization goes, the hybridization of all of the atoms must be the same in all resonance structures. In considering resonance structures, none of the nuclei should be moved in space. Since hybridization is designed to explain the arrangement of nuclei/electrons in space, hybridization cannot change between resonance structures, otherwise nuclei may be forced to occupy new positions.

In practice, this means that certain atoms may appear to be sp3 hybridized but with consideration of resonance structures actually should be treated as sp2. This often means that a lone pair is not in an sp3 orbital as expected, but an unhybridized p-orbital.

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  • $\begingroup$ Thank you for this insight; I keep forgetting that resonance structures are not discrete! $\endgroup$
    – Dissenter
    May 19 '14 at 22:02

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