Determining Bond Strength with Resonance

[joanneyseung22]

Conceptually, how does resonance affect the strength of bonds? I'm not sure how resonance leads to the stabilization of a molecule when multiple-bond characteristics are spread over the bonds in different atoms.

[David S]

Resonance essentially means that the actual model of a molecule is the hybrid of equally favorable lewis structures. One attribute of hybrid structures is that they have characteristics lying in between the attributes of the constituent structures.

Let's take sulfate ion for example. In sulfate ion, the central sulfur atom has a sigma bond between each of the four oxygen atoms, and two additional d-p π bonds with any two surrounding Oxygens. Thus, we can draw resonance structures with only the placement of d-p π bonds differing. So if in some of the resonance structures, a particular oxygen has a single bond, and in the other resonance structures, it has a double bond, we predict that the actual, hybrid structure would result in that oxygen having a bond that has characteristics between that of a single bond and that of a double bond (not as long and weak as a single bond, but not as short and strong as a double bond)
This is true for all the Oxygens in sulfate ion, and thus all the O--S bonds are equivalent single bond/double bond hybrids.

The reason it is more stable this way is because the delocalization of the d-p π bond electrons result in a more symmetric structure with equal length bonds and eliminates the disparity in bond strengths (rather than having two stronger bonds and two weaker bonds, you have 4 equally strong bonds, all of which are stronger than single bonds)

[Mariah Hill]

Will we have to know how to calculate the strength or length of the bonds?

[505194972 3k]

Will we have to know how to calculate the strength or length of the bonds?

No we should not have to know that. I believe in class when professor Lavelle would give us values for the strength/length he mentioned that he had looked up that value.