Question

What are minor resonance structures?

Answer 1

Minor resonance structures are all the resonance contributors that are higher in energy than the lowest-energy contributor.

For example, we can draw three possible contributors for formamide, ${HCONH}_2$

We have to decide which of these is the lowest-energy form. That one will be the major contributor. All the others will be minor contributors.

In order of importance, some rules that enable you to decide are:

$\bullet$ Satisfy the octet rule.

$\bullet$ Minimize charge separation.

$\bullet$ Put negative charge on the more electronegative atom.

Structures III and IV both obey the octet rule, but III has no charge separation. Structure V does not give the C atom an octet.

III is the major contributor. So IV and $V$ are minor contributors.

IV is a more important contributor than $V$, because it obeys the octet rule.

We could say that IV is a minor contributor and $V$ is a very minor contributor.