Download Understanding Molecular Shapes and Bonds: The Role of Resonance Structures and more Study notes Literature in PDF only on Docsity!
(published in CHEM 13 NEWS/January 1993)
What’s the shape of that molecule?
Robert Perkins and Claude Lassigne Kwantlen College P.O. Box 9300 Surrey, B.C. V3T 5H
Part 2. Will that be a single, double or a triple bond?
Having correctly predicted the shape^1 of a molecule, cation, or anion, a problem arises when the student wants to know the actual structure of a molecule which contains O, N, or S atoms. In these situations, two (or more) structures, differing only in the position of the electrons, are often used when the properties of the compound cannot be rationalized through the use of single Lewis structure. Curved arrows can be used to progress systematically from one structure to another; however, the electrons fo not truly shift but are delocalized between the atoms.
For example, two resonance forms may be drawn for ozone (a V shaped molecule via the method introduced in the previous paper):
O
O+
O-^ O-
O+
O
Neither one is a correct structure for O 3 as the experimentally determined bond lengths are the same, rather than the single and double bonds illustrated in the resonance structures. In this case we have a real molecule described in terms of two non-existent molecules. The important thing for the student to realize is that only the shape of the molecule is a measurable property, one cannot "see" whether there is a single, double, or a triple bond between any two atoms in the molecule. Thus we are uncertain about the exact location of all the electrons in the molecule (hardly surprising on the basis of Heisenberg’s Uncertainty Principle).
We have found that stressing to the students that the shape of the molecule is the same for all the various resonance forms assists the students with some of the following.
Example 1.
Which will have the longer N-O bond length: NO2+^ or NO2-?
The nitrogen atom can accommodate only 8 electrons so that we can have several resonance forms based on the following molecular shapes:
O N O O N O O N O
NO 2 +^ - a linear structure
O
N
O
N
O O
NO 2 -^ - a V-shaped structure
As with O 3 , there is a single N-O distance in each of NO2+^ and NO2-; however, in NO 2 -^ the N-O bond order in each resonance structure is 1.5 (3 pairs of bonding electrons shared by two oxygen atoms) while in NO 2 +^ it is 2.0 (4 pairs of bonding electrons shared by two oxygen atoms). NO 2 -^ does indeed have the longer N-O distance 2 - 124 pm versus 110 pm for NO2+. We cannot draw the true structure of either ion, but we can make use of the resonance structures to describe differences between the two ions.
Example 2.
The molecule N 2 O could have two possible molecular arrangements: N-N-O or N-O-N. Using resonance structures, suggest which one is more likely.
Both molecular arrangements will have a linear shape (AX2), and we can draw several resonance structures for each one:
- (^) + + - N N O (^) N N O (^) N N O
N-N-O structure
2-
2- 2+^ - 2+ - (^) 2+ 2- N O N (^) N O N (^) N O N
N-O-N structure
If one considers the formal charges, one molecular arrangement appears more likely: N-O-N always has a 2 +^ formal charge on O, while N-N-O has resonance structures in which a smaller charge separation is present. This would lead one to predict that N-N-O is the correct molecular arrangement of atoms.
One must be careful that students understand the differences between facts and theories when dealing with these types of questions. Purser^3 has recently described the debate in the literature concerning the structure of SO 2. There are six different sets of possible resonance structures (all with the same V-shape) for this molecule; ranging from two sulfur-oxygen single bonds to two sulfur-oxygen triple bonds. The true sutructure, he concluded, was best represented by having two sulfur-oxygen double bonds.
Rather than wasting time drawing multiple Lewis structures, it would be more useful to provide the students with the experimentally determined bond lengths of a series of compounds and allow them to suggest the most likely structure for the compound under consideration.
Example 3. Construct resonance structures for the sulfate ion (SO 4 2-) and use the listed bond lengths^4 to suggest the most likely structure of H 2 SO4. S-O in SO42-^ 149 pm (all the same)
