Resonance

Resonance Structures & Hydbrid Orbitals
15:38 — by Dennis H.

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Key Questions

Resonance refers to the existence of numerous forms of a compound, and is a component of valence bond theory.

Explanation:

Resonance refers to the existence of numerous forms of a compound, and is a component of valence bond theory. Shown below are the two resonance structures of benzene:

These appear to be simple mirror images of each other, but this in fact represents a process that occurs continuously throughout the lifetime of a benzene ring. Observe that benzene is composed of both $\text{C - C}$ single bonds, and $\text{C = C}$ double bonds.

Since a $\text{C = C}$ double bond is stronger than a $\text{C - C}$ single bond, due to the presence of a $\pi$ bond, it follows that the $\text{C = C}$ bonds must have a shorter bond length than the $\text{C - C}$ bonds. The bond length of a $\text{C - C}$ single bond is $154 \text{pm}$, whilst the bond length of a $\text{C = C}$ double bond is $133 \text{pm}$.

Yet the bond length shown in benzene is $139 \text{pm}$. Not only that, but all of the bonds have the same bond length. How can this be? Molecular orbital theory is able to explain this, because resonance is integral to its conception, but valence bond theory must take a different tact if it wishes to explain this.

Recall that $\pi$ bonds result from the extension of half-filled, unhybridised p atomic orbitals from the bonding plane, which can overlap side-on - perpendicular to the $\sigma$ bond. In doing this, the overlapping regions will result in a single $\pi$ bond that contains a pair of bonding electrons. When this happens in benzene, we see an alternating pattern of $\text{C - C}$ single bonds and $\text{C = C}$ double bonds, as shown in the first diagram.

But a p atomic orbital on, say, ${\text{C}}^{1}$, can overlap with, to form a $\pi$ bond, a p orbital on ${\text{C}}^{2}$ or ${\text{C}}^{6}$, can it not? Upon consideration, neither of the two local carbon atoms is any more favorable than the other for this purpose. Because of this, valence bond theory postulates that the structure resonates: it holds in one formation for an instant, before moving onto the next one, then back again, because neither one is more stable than the other.

This actually strengthens the structure, because what appear to be simple $\text{C - C}$ single bonds actually experience double bonded properties instantaneously. The reason for the intermediate bond length in benzene is due to the fact that these bonds are effectively intermediate in identity between single and double bonds.

Resonance plays an important structural role in covalent compounds in accordance with valence bond theory. It can be applied not only to benzene, but also to other compounds such as ozone and carbocations.

• Resonance means that a certain frequency of wave hitting some object is "in synch" with the natural vibrating frequency of that object. If the wave is in synch, it reinforces the natural vibration of the object and can cause the amplitude (amount) of vibration of the object to increase greatly.

A simple example is a playground swing. A swing is a pendulum, and the period of a pendulum is affected only by its length, not by the weight of the person on the swing or by how high the swing swings. (Actually, if you swing really high, the period becomes longer, but for small swings, the period is not affected by how high you go.)

The period means how many seconds it takes to go back and forth once. The frequency, how many times it goes back and forth every second, is the reciprocal of the period, so the frequency is also related only to the length of the pendulum. So a particular swing is always going to have the same frequency, no matter how big the child is or how hard she works to make the swing go high. That fixed frequency is the natural resonant frequency of that swing.

Now if a parent gives a little push every time the swing is on the way down, the child can swing higher and higher. The pushes are in synch with the swing, and they increase the amplitude (height) of the swing's back and forth motion. This is an example of resonance. The pushes (called "driving forces") are in synch with the natural resonant frequency of the swing. If the parent pushed at a frequency not the same as the natural resonant frequency of the swing, the swings would not be reinforced and the child would not swing higher and higher.

Here is a video of a wine glass breaking when someone aims sound waves at it that are at the natural resonant frequency of the glass. This happens because the "pushes" from the sound waves arrive in synch with the natural resonant frequency of the wine glass.

• Yes and no.

When you have several resonance structures for a compound, you draw it as something like this:

However, the true structure of the nitrate ion isn't any of these. Or, to put it more confusingly, it's all of these things simultaneously. What these three Lewis structures serve to do is to show us that all three of the oxygen atoms have some negative charge while the nitrogen has all of the positive charge. Because this is impossible to show with a single Lewis structure, we draw these three and assume that others will know that the real structure is somewhere in between.

OK.

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