Resonance Structures

Don’t move
single bonds.
Only move
pi electrons (double bonds, triple bonds, and lone-pair electrons).
Cations next to a double bond, triple bond, or lone pair.
Push electrons from the double bond, triple bond, or lone pair toward the charged atom.
If lone pairs are directly attached to a double or triple bond,
push the electrons from the lone pair toward the double bond or triple bond.
If a double or triple bond contains and electronegative atom (such as O or N),
you can draw a resonance structure in which the pi electrons move onto the electronegative atom as a lone pair.
If you have alternating double bonds around a ring,
push the double-bond electrons all the way around the ring to reform each of the double bonds between different atoms.
Assigning importance to resonance structures
Those with fewer charges are more stable. The most stable structures have charges on best atom. Better to have complete octets (this is more important that having the charge on the best atom).

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