RESONANCE MOTIFS+CRS

What is Motif 1?

Motif 1 Requirements:

atom X has

a) 4 neighbors,

b) at least one neighbor is a lone pair,

c) at least one neighbor, atom Y, that is part of a π bond.

Consequence: Atom X is sp² hybridized, with its lone pair placed in the p-orbital, this p orbital is parallel to the p-orbital on the neighbor  Y that is part of the π bond (to atom Z).  The electrons spread over all three parallel p-orbitals. One of the Resonance Structures has X with a lone pair and neighbor Y with a π bond to atom Z. The second Resonance Structure has a π bond between atom X and atom Y, and atom Z having a lone pair. These two structures will have different formal charges on atoms X and Z. 
Example:      

What is Motif 2?

Motif 2: a cyclic array of p-orbitals, i.e. the multiple π bonds in a planar benzene (aromatic) ring system, require multiple resonance structures to correctly describe the ACTUAL structure. All bond lengths in benzene are all the same, corresponding to a bond order of 1.5 instead of alternating 2-1-2-1-2-1.
Example:    

What is Motif 3?

Requirements:

atom X has

a) 3 or 4 neighbors,

b) at least one neighbor is a lone pair,

c) is bonded to atom Y that has only 6 valence electrons (most commonly a C⊕ or neutral B).

Atom X and Y will share a π bond using the two electrons from the lone pair on atom X. This gives atom Y a complete octet.  The resonance structure without the π bond does NOT contribute to the resonance hybrid. 
Example:  

 
left Resonance Structure is not included in the Resonance hybrid. 
right Resonance Structure best represents the ACTUAL STRUCTURE of this compound.

What is Motif 4?

Motif 4: A π bond adjacent to an atom with an incomplete p orbital (most commonly a C⊕ or neutral B) will form a π bond so that a previously incomplete valence shell is now full, but leaving behind an incomplete valence shell.  If the original structure was contributing, the new structure will also be contributing.  
Example:  

forming six more equivalent CRS!

How do you identify CRS?

1) Complete as many valence shells as possible. This is synonymous with making as many bonds as possible.

2) Second row atoms can have formal charges of -1, 0, +1 only!

How do you compare/rank CRS?

1) A structure with fewer formal charges is probably more contributing

2) If you must have formal charges, put them on appropriate atoms (most electronegative atom gets the -), carbon avoids having a FC id a heteroatom like N or O could host it instead.

What is the first ground rule for drawing/interpreting Resonance Hybrids?

1) The multiple Resonance Structures in a Resonance hybrid identify multiple pairs of adjacent atoms that share π-bonding in the ACTUAL STRUCTURE of a molecule.  The number of adjacent atoms with parallel p-orbitals in the ACTUAL STRUCTURE (represented by the Resonance Hybrid)  is larger than the number of atoms with parallel p-orbitals (π-bonds) in each individual Resonance Structure. Electron delocalization (i.e. spreading) across a larger π-system lowers the electrons' (and the molecule's) energy.

What is the second ground rule for drawing/interpreting Resonance Hybrids?

2) There is only one ACTUAL STRUCTURE of the molecule. In that ACTUAL STRUCTURE..    
      i) each atom has one hybridization -- that atom hybridization is the same for every Resonance Structure in the hybrid.
     ii) each atom is sigma-bonded to neighboring atoms -- that set of sigma bonds is the same in every Resonance Structure
    iii) each bond has one bond order -- one Resonance Structure may indicate X-Y (single bond); a second Resonance structure may indicate X=Y (a double bond). ⟶ In the molecule's ACTUAL STRUCTURE, atoms X and Y have a bond order between single and double.  
     iv) each atom has one formal charge -- one Resonance Structure may indicate that atom X is neutral; a second Resonance structure may indicate that atom X has a -1 formal charge. ⟶ In the molecule's ACTUAL STRUCTURE, atom X has a formal charge between 0 and -1.

What is the third ground rule for drawing/interpreting Resonance Hybrids?

3) Resonance Structures differ by which atoms share π-bonds. The atoms that participate in this extended π-bonding must be sp or sp² hybridized, with the unhybridized p-orbital parallel to a neighboring atom's p-orbital with which it shares π-bonding in the ACTUAL STRUCTURE (and in at least one Resonance Structure).

What is the fourth ground rule for drawing/interpreting Resonance Hybrids?

4) Resonance Structures differ by which atoms share π-bonding.  The ACTUAL STRUCTURE has partial π-bonding between atoms that show π-bonding in some but not all resonance structures.  Atoms that share a π-bond in every Resonance Structure have a normal, full π-bond.