Resonance Chemistry

Figure 14.

As you can see, the first CO2 structure is more stable because it has a zero formal charge on all atoms, while the second leaves an unacceptable charge on the two oxygen atoms. In general, the Lewis structure with the set of formal charges closest to zero will be the most stable.

RESONANCE CHEMISTRY

A discussion of resonance chemistry can be had by looking at the CO3(2-). This has carbon in the middle, a double bond with one oxygen and a single bond with 2 oxygens and an overall charge of 2-. The big question is this: Which oxygen molecule gets the double bond and which get a single bond? This can be explained in terms of resonance. When more than one Lewis dot structure can be drawn for the same molecule, this is said to have “resonance”.

What this means is that, for CO3(2-), the charge is rapidly shifting from one place to another in a sort of blur that gives each CO bond equal stability, so that a third of the time, the double bond will be on one of the three oxygen atoms, so that there is equal sharing of this double bond. Instead of a -1 charge on two of the atoms, there will be a 2/3 charge on each oxygen atom on average. Figure 15 shows the resonance of CO3(2-).

Figure 15.

The doubles-sided arrow will show that these structures are equivalent. There is not a true -2/3 charge on each atom but, experimentally at least, there is no difference between structures A, B, and C and the experiment to find these structures indicates that these structures as written do not exist.

You can draw resonance structures by drawing all of the possible structures and charges on a molecule; however, this can be cumbersome. The only difference between the Lewis structures of these molecules is the placement of the electrons. The atomic position is exactly the same.

One can use a curved arrow in order to indicate that an electron shifts between one atom or another. In the example shown in figure 16, there is a curved arrow that indicates the shifting of an electron pair in the ethylene molecule. This is called “electron pushing”.

Figure 16.

In drawing resonance structures, all structures must have the same number of valence electrons with no destruction or creation of electrons. The octet rule needs to be obeyed at all times. In other words, you cannot have five bonds around carbon or more than one bond associated with hydrogen. Nuclei cannot change positions in a resonance structure (only the actual electrons).

Ozone is a typical resonance structure. It involves three oxygen molecules together that have a double bond between one oxygen and a single bond between the other oxygen. This, however, does not stay stable and the double bond shifts from one oxygen molecule to another. Figure 17 shows the ozone molecule in its resonance arrangement:

Figure 17.

You can try this yourself as you discover that, like ozone, molecules like NO2 - will have a resonance structure as well. In short, resonance structures are a simplified way of describing molecular orbitals that extend to cover more than two atoms. The electrons will shift from one place to the next and will be “averaged out” over the course of two molecular bonds.