Resonance Between Three Atoms

The next common patterns you will see in organic chemistry

1. Atom with an incomplete octet next to a π bond.
   
   For example in the following case we have a pi bond adjacent to a C atom with a + charge.  The + charge is a clue that it does not have a full octet.  It only has 6 electrons surrounding it.
   
2. Atom with a lone pair of electrons next to a π bond. 
   
   Here we have a pi bond adjacent to a C atom with a negative "-" charge.  The "-" charge is a clue that it has a lone pair and a full octet.
   

Atom with Incomplete Octet next to π bond

It's important to be able to draw other important resonance structures, when needed.  Here is an example using generic atoms.

A good example of this is the following propene cation (allylic cation).

Which carbon has the greatest positive charge?  ;-)

The resonance hybrid would look like the following, in which the charge is shared between each terminal carbon.  Each C-C bond is also the exact same length, and their length is somewhere between that of normal C-C single and a C=C double bond.

The propene cation's electrostatic potential map is shown below.  Each carbon is identical and each C-C bond is identical, as our simple resonance would predict.  Blue indicates more positive charge.  Suppose you were a nucleophile (Nu^-), which carbon would you attack? 

 Atom with lone pair next to π bond

When the atom next to a π bond has a lone pair of electrons, obtaining the other resonance structure is slightly different.  In this case there are two arrows and one of the arrows starts on a lone pair.

The allylic anion shown below is a good example.

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