Guidelines for drawing and working with resonance contributors

Learning to draw and interpret resonance structures, there are a few basic guidelines to help avoid drawing nonsensical structures. All of these guidellines make perfect sense as long as we remember that resonance contributors are merely a human-invented convention for depicting the delocalization of pi electrons in conjugated systems.  When we see two different resonance contributors, we are not seeing a chemical reaction! We are seeing the exact same molecule or ion depicted in two different ways. All resonance contributors must be drawn as proper Lewis structures, with correct formal charges. Never show curved 'electron movement' arrows that would lead to a situation where a second-row element (ie. carbon, nitrogen, or oxygen) has more than eight electrons: this would break the 'octet rule'. Sometimes, however, we will draw resonance contributors in which a carbon atom has only six electrons (ie. a carbocation). In general, all oxygen and nitrogen atoms should have a complete octet of valence electrons.

1. There is ONLY ONE STRUCTURE for each compound or ion. This structure takes its character from the sum of all the contributors, not all resonance structures contribute equally to the sum.

2. Atoms must maintain their same position. 

3. Only e- move !

4. All resonance contributors for a molecule or ion must have the same net charge. 

5. Recognize which electrons can participate in resonance

a) unshared e^- pairs or radicals

b) pi bond electrons

6. Recognize electron receptors

a) atoms with a positive (+) charge

b) electronegative atoms that can tolerate a negative charge

c) atoms which possess delocalizable electrons - see #4 above

7. Common electron flow patterns

a) move pi e^- toward positive (+) charge or other pi bonds

b) move non-bonding e^- pairs toward pi bonds

c) move single non-bonding e^- toward pi bonds