CHEMISTRY COMMUNITY

Can someone explain how to determine the bond lengths given that the single bond length is 140pm and the double bond is 120pm in a nitrate structure?

First, realize that nitrate's lewis structure has resonance and is the reason why the bond lengths are averaged. This blend of single and double bond lengths accounts for this averaged length.

Because there is resonance, you can average the two. The average of 140 and 120 is 130, therefore the bond length should be 130 rpm

Since the structure of nitrate has both a double and single bond, we have to understand that their is resonance involved. Also, remember that double bonds have shorter bond lengths than single bonds because double bonds are stronger. Resonance affects the strength of bonds because it spreads multiple-bond character over the bonds between atoms so single bonds are strengthened while double bonds become weaker as a result of the bond lengths being averaged.

Due to resonance, the most stable structure of the nitrate will be an average of the bonds nitrate holds which is both a single and double bond. As an overall molecule, nitrate's structure will even out and be an inbetween version of a single and double bond. Therefore, the average of 120 and 140 is 130.

There is resonance therefore, you take the average of the two.

You can take the average of the two

If a structure has multiple bonds then the total molecule will have bonds the average length of all the present bonds. A molecule that has resonance has multiple possible bond placements in a Lewis structure, but this simply translates to having bonds that are averaged out over the different bond lengths. If you have a 120pm and a 140pm set of bonds in a molecule, the electrons will be held an average of those lengths or somewhere around 130pm.

So resonance creates a more stable bond, but how do lone pairs affect the strength of the bonds within a molecule?

Because there is resonance here, the bond length is the average of the bond lengths of the two structures.

You can add the bond lengths together and divide by the total amount of resonance structures to find the average bond length.