In chemistry what is a resonance?

Answer 1

A resonance structure is an alternate way of drawing a Lewis dot structure for a compound. For some molecules, there are multiple ways to draw a Lewis dot structure that still satisfy the rules (for instance, having the correct total electron count and satisfying the octet rule on each atom).

Benzene is a classic example where a resonance structure is used. See the Web Links to the left for a diagram of benzene's two resonance structures. Note that in both cases, there are alternating double and single bonds between the six carbons. What changes in the two structures is which bonds are single and which are double bonds. In reality, all the bonds in benzene are identical. They are neither single nor double bonds, but something like a "one-and-a-half bond."

There is a common misconception that in reality the molecule is somehow alternating between the two structures. It is not.. This is extremely important to understand! Resonance structures are simply a tool to make up for the fact that Lewis dot structures are not the best tool for describing bonding in molecules (they are very good, just not perfect!). Using resonance structures is a kind of band-aid to patch up this shortcoming of Lewis dot structures (for a better representation of bonding, you must use molecular orbital theory which requires quantum mechanics!).

So if the is not alternating between the two structures, what's going on?! In fact the molecule's structure is always the same. The actual bonding the molecule is a mixture of the different resonance structures all the time. To see this, you have to look at the different structures and imagine blending them together to give one single structure which has contributions from each structure. So the bonds in benzene are not going back and forth being single and double bonds. Not at all! Not even really quickly. Instead the bonds are ALWAYS this average bond which is a bit of both single and double.

Note also that not all resonance structures contribute equally to what the molecule really looks like. For benzene, there are two structures, and the real molecule is a perfect mixture of both structures, 50% each. However, the are some molecules for which you can draw more than a dozen structures! In this case, you might have one structure that is dominant, and the real molecule's structure is close to that, and the other structures only contribute a little bit. In other words, the real structure is a weighted average of all the different resonance structures, but the weighting for each structure depends on the details of that structure, and some have very little weighting.

How do you tell what structures are more important that other ones? Only if the structures are identical by symmetry, like for benzene, do they contribute exactly the same amount. Another way to say this is that if you can convert between two structures just by rotating the drawing around, they are the same by symmetry. However, sometimes they are not the same at all. Here is how you tell. A better resonance structure has the following properties:

-- bond is maximized

-- formal charges are minimized

-- negative formal charges are carried by the most electronegative atoms

There is no way to predict exactly how much each structure will contribute (except when two are equal by symmetry). But you can put them in order based on the three rules above (and they are listed in order of importance).

Answer 2

It's a way of showing the polarity of a bond as well as partial charges on atoms. None of the drawings of a resonance structure actually exist, however the drawing helps show where partial negative and partial positives exist.