Resonance Structures

Sometimes we end up with a line/dot structure where there is a choice of 2 or more places to put a double bond surrounding a central atom. When the two possible locations are identical - meaning to the same atom, just in two or more different locations, we drawn resonance structures. Draw BOTH structures (or three, etc...) that represent the two extreme cases and put a double headed arrow between them. This is the way we show resonance. Ozone, O₃, is exactly this case. If you use the S = N - A thing, the number of electrons to be shared is 6. But there are only 2 positions possible. So the double bond is placed in both positions and resonance is shown.

As shown in the last section, we find it extremely helpful to actually show the valence shell electrons (the s and p ones) as dots surrounding the element symbol. Put your new found knowledge of electron configurations to work for you. Count those valence electrons - yes, the outermost s and p ones. I'll give you a big hint, the number of valence electrons EQUALS the ones-digit of the group number of the periodic table (skipping the d-transition metals). So group 1 and group 2 have 1 and 2 valence electrons. Now jump over to the p-block and you have groups 13-18. That will be 3 valence electrons all the way up to 8 valence electrons. You just put the dots symmetrically around the symbol and you've got it. Here is the electron dot "formula" for the 2nd row elements.

In actuality, both those O-to-O bonds are identical with a bond order of about 1.5. This means the bond is more than a single bond, but less than a double bond. One could say it is the average between 1 and 2 - which is 1.5. Since we can't really split electrons in half and we STILL do our best to obey the octet rule and hund's rule and the Pauli exclusion principle (all those quantum things we learned in atomic theory) - we compromise by drawing the extreme cases and then KNOW the real molecule is an average of the extremes. The fact is that the double bond (the second pair of electrons in the bond - or the second line) is split 50/50 between the two locations in the molecule of ozone. When electrons are in two or more places at one time, we call them delocalized electrons and we show their "realm" via resonance structures.

Benzene C₆H₆

Bezene is a cyclic compound - specifically it is a 6-membered ring that is a perfect and flat hexagon. It is a very recognizable structure - heck, there is even a unicode character for it → ⌬ and even ⏣. So what's the deal with the hexagon with the circle in it? Well, it has to do with resonance. Here is a graphic I made to take you through the...

Evolution of the Benzene Symbol ⏣