Ozone is a symmetric molecule, so I and II contribute equally to our picture of this molecule.
Most resonance hybrids are not symmetric, however, and their contributors do not play equal roles.
Formamide is a superposition of III and IV, but its structure and properties more closely resemble III: the atoms carry only small charges, the CO bond is closer to a double bond, and the CN bond is closer to a single bond. In this case, we say III is the major contributor and IV is the minor contributor.
Ambiguous terminology. "Major" and "minor" are still used when a molecule is a superposition of three or more contributors. In these situations, "major" can be applied to all of the larger contributors, and "minor" to all of the lesser contributors. For example, formamide is sometimes viewed as a superposition of three contributors:
V plays a much smaller role than III or IV, so we might refer to V as a minor contributor, and III and IV as the major contributors.
Sometimes, though, we want to keep the relative roles of all three contributors clear. In this case, we use the following terminology:
Ranking contributors. There are several ways to identify major, minor, and very minor contributors:
- Rank by fit. If you have information about a molecule's characteristics, then you can rank contributors by their ability to match these characteristics. A major contributor, like III, provides the best match. A minor contributor, like IV, provides a weaker match, but adds important information to the picture. A very minor contributor, like V, adds very little to the picture and are normally ignored.
- Rank by energy. Each contributor corresponds to a unique electron pattern, or wave function, for the molecule. The lowest energy wave functions will always be major contributors. Higher energy wave functions will be minor contributors, and their importance will diminish as their energy rises. High energy wave functions are normally ignored.
- Ranking rules. Organic chemists have also devised a standard set of rules for ranking contributors (e.g. Carey 5/e p. 26, Table 1.5 Rules 3 & 4). These rules provide, in effect, a way of estimating the relative energy of different contributors.