Question

What is a major resonance contributor?

Answer 1

A major resonance contributor is one that has the lowest energy.

We can often write more than one Lewis structure for a molecule, differing only in the positions of the electrons.

Each individual structure is called a resonance contributor.

The most stable structures contribute most to the resonance hybrid. They are called the major resonance contributors.

In order of importance, a major contributor must have:

1. The most atoms with complete octets.
2. Any formal charges on the atoms most able to accommodate them.
3. The most covalent bonds.
4. The smallest number of formal charges.

Example 1

You can write three possible Lewis structures for formaldehyde, H₂CO:

The lowest energy form is I, because every atom has a complete octet. It is the major contributor.

II is lower in energy than III, because the negative charge is in the more electronegative O atom.

The major contributor is I.

Example 2

You can write two structures for the conjugate acid of formaldehyde.

The first structure is the major contributor, because every atom has a complete octet. This is despite the fact that the positive charge is on the more electronegative O atom.

Example 3

We can write two contributors for the conjugate base of acetone.

All atoms in C and D have a complete octet. But D has the negative charge on O, so D is the major contributor.