1. Resonance structures must have same connectivity.
QUESTION:
Which of the structures shown below are resonance structures? A. Structures I and II, B. Structure II and IV, C. Structures I and III, D. Structure II and III
SCRIPT: 3-7-1
Which of the structures shown below are resonance structures? A. Structures I and II, B. Structure II and IV, C. Structures I and III, D. Structure I and IV
CLICK
Resonance structures refer to different ways of distributing electrons, for atoms that are connected the same way.
So, the first thing we look for is a pair of structures where the atoms are connected the same way.
For a triatomic molecule, it’s fairly simple.
Note that in structures one and three, Sulfur is between Carbon and Oxygen
HIGHLIGHT S in both structures
We say that the connectivity of the atoms is the same
So, structures one and three COULD be resonance structures. ….
So, choice C is a possible answer at this point.
CALLOUT Put “?” next to choice C.
In structures two and four, Carbon is between oxygen and sulfur.
Choice B is also a possible answer at this point…
CALLOUT Put “?” next to choice
We can rule out choice A. Structures one and two cannot be resonance structures --- the atoms are connected differently.
CROSS OUT choice A
We can also rule out choice D. Structures two and three have different connectivities.
CROSS OUT choice D.
How do we decide between B and C?
For two structures to be considered resonance structures, they must have the same total number of valence electrons.
Resonance structures differ only in the way the electrons are distributed.
Let’s examine structures one and three…
Structure one has sixteen valence electrons…
Eight
Plus eight
CLICK equals 16.
Structure three, on the other hand, has only twelve valence electrons….
Six
Plus six
Equals twelve.
Therefore, structures 1 and 3 are not resonance structures. In fact, structure three is not even a valid Lewis structure.
The sulfur and oxygen do not folow the octet rule.
Obviously structures 2 and 4 are resonance structures.
Structure two has 8
CLICK plus 8
CLICK or a total of 16 valence electrons
And structgure 4 has
CLICK or a total of 16 valence electrons as well.
The correct answer is B.
PAUSE END RECORDING
Resonance structures must have same connectivity.
Resonance structures are just different ways of distributing valence electrons –-- must have same total number of valence electrons.

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08