Selecting The Most Stable Resonance

[Alpha Centauri]

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Hey guys. What is the best way to reason through problems asking to select the resonance structure that is most stable? I have ALWAYS had issues with this throughout Ochem

 

[klownzo]

These questions always involve the octet rule. Just choose to have a + charge on a N or O for example. That way all the carbons will have a filled octet

 

[confidentandgood]

What I always go by (from the 5th edition Jones textbook, pp. 28-30):

1. The more bonds in a resonance form, the more important (stable) a contributor the form is.
2. Separation of charge is bad.
3. In ions, delocalization of electrons (distributing them over as many atoms as possible) is especially important.
4. Electronegativity is important. A negative charge should preferably reside on an electronegative atom.
5. Equivalent resonance forms contribute equally to a resonance hybrid.

And as a side note: all resonance forms for a given species must have the same number of paired and unpaired electrons.