Resonance and Delocalization - Organic Chemistry Concepts: An EFL Approach - page 60

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Therefore, the effect of these charge-separated resonance forms on the overall hybrid
is very small. Compare this to Program 9A in which the propanone situation is very
different.
Here, the electronegative oxygen makes the dipolar resonance form more acceptable.
However, the dipolar resonance form, in which carbon has an incomplete octet, has a
higher energy and contributes less to the hybrid.
Draw a system that shows equal resonance forms.
C One possible example is the nitromethane molecule.
Because these two resonance forms are equal in structure, they have the same
energy. Experimental results show that both N-O bonds are equal in length. They are
shorter than a single N-O bond, but they are longer than a double N-O bond. In this
case, the resonance hybrid is a simple average of the two resonance forms.
Note that curly arrows are used to switch between resonance forms by moving pairs
of electrons between atoms and adjacent bonds. The atoms that receive or give these
pairs only gain a single charge. The reason is that these atoms only get or lose a
share in one of the electrons of the pair. Go back to the examples mentioned earlier
and check this for yourself.
In general structure drawing, it is common to leave out the lone pairs. Do not forget
that they are still there and that they must be included to help with electron counting.