Insignificant Contributors

Some resonance forms do not contribute to the true structure and are insignificant.  They often have more than two formal charges on the functional group.

Insignificant resonance structures contribute less to the overall structure because they are less stable than the major contributors. Here are some things that can make a resonance structure insignificant:

• Having too many formal charges: The more formal charges (positive or negative) a structure has, the less important it is.
• Violating the octet rule: Structures, where atoms have less than a full octet (especially for elements like oxygen or nitrogen), are usually insignificant.
• Having radicals: Structures with unpaired electrons (radicals) are generally less stable than those without them.

Too Many Formal Charges

For example, consider the acetate anion below.  Acetate is the conjugate base of acetic acid.  Structure B has 3 atoms with formal charges and would be considered an insignificant resonance structure and would not contribute to the true structure.

Violating the Octet Rule

As a second example consider the imine below.  Movement of the π bond by the green arrow produces the resonance hybrid on the left.  The red arrow produces the insignificant structure on the right (negative charge not on electronegative O atom).  This also results in the N atom not having an octet of electrons.

Creating Radicals

Finally, it would be bad form to split a covalent bond into unpaired electrons.  Resonance structure E below would be an insignificant contributor.