Using Formal Charge To Determine Major And Minor Contributors (Among Other Things)
Now that we know how to determine the Formal Charge, we can decide what is a Major Contributor and what is a Minor Contributor when it comes to structure.
The Major Resonance Contributor is the most stable resonance structure that exists for a molecule. The other resonance structures will be Minor Contributors.
In general, the Major Resonance Contributor:
has octets on as many atoms as possible
has reduced charge separation on the molecule (formal charges of 0 are best)
has negative charges on the more electronegative elements
has positive charges on the less electronegative elements
places the charge of carbo-cations on the more substituted carbon atom
places the charge of the carbo-anions on the less substituted carbon atom
Knowing this, the first thing you are going to want to do is look for something that does not have Formal Charges. If only one resonance structure has a Formal Charge of 0, that is the resonance structure that will be your Major Contributor, and the rest will be Minor Contributors.
If there is more than one resonance structure with a Formal Charge of 0, then you will have to look at the other bulleted points and determine which meets the criteria of a stable resonance structure best. This will be your Major Contributor. The rest will be Minor Contributors.