Lecture #12
  Text: Section 13.11
  CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Lecture Outline Molecular Structure (Lewis structures)

Resonance

Equivalent preferred contributors

Bond order
Benzene is the cyclic hydrocarbon C6H6. The two Lewis structures drawn here look to be structural isomers. On the left, the chlorines are adjacent to a double bond. On the right, they are adjacent to a single C-C bond. Yet only one isomer exists. Why? The answer lies in the phenomenon called "resonance."
We could estimate the energy involved in the reaction shown by looking at bonds broken and bonds formed during the electrons' rearrangements. (These energies are called reaction heats and will be the subject of further discussion in the next lecture.)
But the value obtained differs significantly fromwhat is actually measured. Why? ..."Resonance!"
Despite a Lewis structure indicating alternating single and double bonds...alternating long and short carbon-carbon separation distances, all C-C bond lengths are identical. Why?..."Resonance!"
What, then, is resonance?
Resonance is condition associated with two or more arrangements of valence electrons. We will be concerned with those that give equivalently preferred Lewis structures.
 First illustration of resonance among equivalent Lewis structures. The Lewis structure is represented by this mixture; not by the three arrangements, but by a single arrangement of electrons too complicated to be drawn on one picture, and so we resort to symbolizing that one picture as a superposition of three pictures we can draw.  
Previously, we had single, double, and triple bonds corresponding to bond orders of one, two and three, respectively. But with resonance structures, we need a modification of our definition of bond order to accommodate intermediate possibilities.
The carbon-oxygen bond order in the carbonate ion is 1.33 for each of the three CO bonds.
 For benzene, the carbon-carbon bond order is exactly halfway between a pure single bond and a pure double bond.  
Any reference to a complete, preferred Lewis structure will imply that formal charges must be indicated where necessary and all resonance contributions of equivalently preferred structures are needed as well.
The nitrate ion preferred structure.
Illustrating what is meant by equivalent structures using an example where structures are not equivalent.
Finally, another "heads up" example.
A sample question about Lewis structures and bond orders for some oxide ions of astatine (Z=85).