What are major resonance structures?
WHAT IS A MAJOR RESONANCE STRUCTURE?
The most stable structural "snapshots" of a molecule's electron distribution is its major resonance structure. Let's suppose that we examine the resonance structures of urea,
The curved arrows indicate the movement of electrons that lead to a new resonance contributor. We start at I and continuously move clockwise.
FORMAL CHARGES & ELECTRONEGATIVITIES
First, see how structures III and IV do not have minimized formal charges? Although there is net charge cancellation, nitrogen, being less electronegative than oxygen, tends less to pull electrons towards it; therefore, the major (most stable) resonance structure gives the most electron density to oxygen.
That means I is the major resonance contributor. That automatically means III and IV are the minor resonance contributors (second-most stable).
Also, notice how structures III and IV are actually identical; just reflect them along a vertical axis and they are the same. Those would be known as degenerate structures, i.e. structures that have the same energy as each other.
#E_"III" = E_"IV"#
Finally, see how structure II leaves carbon without an octet? You may have learned that atoms prefer to satisfy the octet rule.
Because of a lack of that here, structure II is the most minor resonance contributor (least stable) of these four.
STABILITY & RESONANCE CONTRIBUTIONS
In the end, we have this spectrum of stabilities and resonance contributions: