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Now that we are familiar with pi bonds, we can connect back to resonance.

Another way to describe resonance is as delocalized pi bonding . Only electrons in p orbitals can be delocalized in a pi system.

Let's use the amide functional group shown below as an example. The experimental H-N-H bond angle is observed to be 121.6 degrees. How do we explain this? What is the geometry? What is the hybridization of the nitrogen atom? This is another example of where we need to consider the available theories in order to explain the experimental observation.

According to VSEPR, we would predict a tetrahedral electron domain geometry and angles less than 109.5 degrees (much like ammonia, NH_3 ). This disagrees with our experiments!

Looking at the amide from a simple valence bond approach where unhybridized atomic orbitals are used isn't satisfactory either. This theory would predict 90 degree bond angles if the bonds involved the overlap of pure p and s orbitals.

Okay, okay... What if we hybridize the orbitals? That nitrogen atom has 4 domains, so if we hybridize the 2s and three 2p orbitals, we get four equivalent sp^3 hybrid orbitals. These will arrange themselves in a tetrahedral geometry to minimize electron-electron repulsion. That takes us back to the 109.5 degree bond angles...

Ugh, now what?! Go back to the data. The bond angles are about 120 degrees. This corresponds to a trigonal planar geometry which results from sp^2 hybridization (mixing of the 2s and two 2p orbitals). Perhaps the nitrogen atom contributes sp^2 orbitals to form the bonds. What happened to the remaining p orbital that wasn't hybridized? It's still there! That orbital holds the lone pair.

So, why is this system an exception to several of the above mentioned theories? The answer is resonance! When electrons are delocalized, they are lower in energy, and the molecule is more stable. In order for electrons to be delocalized via resonance, they must be in a p orbital. Remember the four patterns of resonance? Each of those patterns involves pi electrons. An atom or molecule wants to reach the lowest energy state, so it will adopt the geometry and hybridization to achieve it.

The take home message: be on the lookout for those 4 patterns of resonance! If electrons on an atom are delocalized (involved in resonance), they will be in a p orbital. That means the atom must be sp^2 or sp hybridized.