Resonance

You may or may not have thought about this issue, but a decision was made in the LDS for NO₂ on the previous page that has no justification whatsoever. When one pair was moved in from an oxygen into a double bond with the nitrogen, that pair was moved from the oxygen on the left side of the molecule. It could just as easily have been moved from the oxygen on the right. This leads to two possible LDS's.

It is tempting to think that these are the same structure. After all, oxygen is oxygen. In other words, one structure is just the other one viewed from the back (or in a mirror). However, you will remember that not all oxygen atoms are the same. Some have an extra neutron or two. Thus, if the two oxygen atoms are different isotopes there is a distinct difference between the two structures.

In the first structure, the heavier atom of oxygen has the double bond, while in the other, the double bond involves the lighter oxygen. Knowing, as we do, that a bond is an attraction that holds atoms together, we can easily see that a double bond, with its extra electrons involved in the attraction would be harder to pull apart (have a larger bond energy) and that such a bond would be shorter since the atoms are pulled together with more force.

Logically then, if we could measure the strength of the two bonds, or their length for that matter, we would be able to determine which is which.

However, when the bonds are measured, they are both exactly the same length and exactly the same strength. To make matters worse, both bonds are stronger than single bonds, but are weaker than double bonds. Both bonds are shorter than single bonds, but longer than double bonds. Both bonds seem to be the equivalent of 1 and ½ bonds.

NOTE: This means that the Lewis Dots Structures are NOT correctly representing the truth!

At first it was theorized that the electrons were moving back and forth between the two structures, so that the structure was resonating between the two extremes. As a result, this property became known as resonance. This is unfortunate, since we have since figured out that the molecule is NOT resonating. In fact the real molecule is never either structure, but rather lies somewhere in between, with the electrons spreading themselves out over the whole structure in a way that LDS's cannot adequately show.

There are several commonly accepted ways to show a LDS for a molecule such as NO2 that demonstrates resonance. One is to represent the ½ part of each bond as a dashed line. Generally this is accompanied by a dropping of the outer electron pairs for convenience.

The other way to represent this molecule in a LDS is to draw the two (or sometimes more) possible structures and to place a double-headed arrow between them. This dates from the time that it was believed that the molecule was resonating between the various possibilities.

The first of these two choices, is more accurate when it comes to representing the actual bonds that exist between the atoms, although it must be acknowledged that the dashed line can sometime be half of a bond, and at other times a third of a bond.

The second method is more honest in terms of the electrons present in a molecule.

It will be up to your instructor to determine how he or she expects you to draw the LDS of such molecules.

Your Turn: (answers are here)

Try the following molecules and ions, but be careful. Some involve more than two possibilities. In addition, don't forget that ions need to be shown in large square brackets with their charge. (the answers are here)

SO₂
SO₃
nitrate
nitrite
acetate
oxalate

VSEPR Theory