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Sometimes more than one Lewis structure will be possible for a molecule. When several "legitimate" structures are possible, the true structure is a mix or hybrid of all the contributing structures. (A legitimate Lewis structure may not be the best structure, but will follow two basic rules: 1) Do not exceed the octet for second-period elements and 2) have no more than two charged atoms with magnitudes no more than \pm 1. Of course, there are exceptions, and one is the NO _3^- ion. It will have three formal charges.) The hybrid structure will show more accurately how electrons are delocalized over several bonds or atoms. Electron delocalization influences bond order, bond length, and bond strength. It's important to note that resonance is a stabilizing effect - when electrons can be spread over a larger space, they are at lower energy.

There are three rules for drawing resonance structures. Resonance structures will differ only in their arrangement of electrons, so their framework must be the same. The second is not to exceed the octet for second period elements. (It is okay to have fewer than 8 electrons in many cases.) The final rule is to never break single bonds. Electrons in the second or third bonds are the only ones that can be moved when drawing contributors.

Resonance involves the redistribution of electrons in lone pairs or double (or triple) bonds. Take a look at the video below that introduces some of the basic concepts of resonance. From now on, you will need to consider resonance when determining bond order, bond length, and bonds strength.