Bonds - Order, Lengths, and Strengths

Every covalent bond is slightly different depending on the two atoms that make the bond and the specific environment that the bond is in. Since we measure bond distance (length) from nucleus to nucleus, it makes sense that bigger atoms have longer bond lengths and that is true. But there ARE some trends and patterns that one should learn, memorize, and take note of.

Bond Order

Bond order is just a way for us to put a "scale" on bond types. It is very straight forward - a single bond has a bond order of 1, a double bond has a bond order of 2, and a triple bond has a bond order of 3. So when we ask for bond order, we are wanting to know about what type of covalent bond is present. We even sometimes ask for "overall" bond order in a molecule. The overall bond order is all the bond orders added up to a grand total. Like in methane (CH₄), there are 4 single C-H bonds each with a bond order of 1. The overall order for methane is 4. Pretty straight forward.

Fractional Bond Orders Thanks to the concept of electron pair delocalization and resonance, there are certainly situations where you get fractional bond orders and not just the friendly three integer ones of 1, 2, and 3. Most common is when an electron pair in a double bond is delocalized across 2 or 3 regions in a molecule. Like in ozone - it has two resonance structures. The bond order is NOT 1 for one bond and 2 for the other. Remember, resonance shows the extremes that are to be averaged for the better picture of bonding. The two bonds are identical and the bond order is 1.5 for each. The overall bond order is 3. What is nice about overall bond order is that it really IS an integer because you can just count all the bonding pairs in a molecule and those will be an integer number. In ozone the total number of bonding pairs (lines) is 3. Those are only in 2 regions though. Therefore we average and get 3/2 or 1.5 bond order for both bonds in ozone.

Bond Lengths and Strengths

Anytime two nuclei are pulled closer and closer to get a stronger bond, the bond gets shorter and shorter. Sort of a common sense thing... the coulombic pull on the bonding electrons pulls the nuclei closer to each other which means a shorter bond. Simultaneously, the more they pull, the stronger the interaction and bond strength. Bond order fits right into this as well. A double bond between two atoms is stronger and shorter than a single bond between the same two atoms. A triple bond is even stronger/shorter. And, you guessed it, a bond order of 1.5 (like in ozone) is stronger than a single bond, but weaker than a double bond. Below are average bond lengths and strengths for all types of carbon-to-carbon bonds.

You might notice that the double bond for two carbons is not actually double strength - it's more like 1.5× a single, and a triple bond is more like 2.5× a single. So in this case, a triple bond isn't triple strength, but it is definitely a lot more. In other instances, a double and a triple are more than double and triple strengths of the single - like with C–N bonds. So don't try to read too much into the whole "double" and "triple" when dealing with strengths. Know the strength hovers around that area but isn't exact.

The lengths are also a bit of a subtle change... a double bond is about 90% of the length of a single bond and a triple bond is about 80% the length of a single bond. And remember, I'm comparing C-C bonds here. Certainly things are a bit different when you compare different atoms to different atoms. None-the-less, the trends are present and you should know the general trends.