Lecture 24: VESPR • Reading: Zumdahl 13.13 • Outline – Concept behind VESPR – Molecular geometries

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- Slide 1
- Lecture 24: VESPR Reading: Zumdahl 13.13 Outline Concept behind VESPR Molecular geometries
- Slide 2
- Resonance Structures We have assumed up to this point that there is one correct Lewis structure. There are systems for which more than one Lewis structure is possible: Different atomic linkages: Structural Isomers Same atomic linkages, different bonding: Resonance
- Slide 3
- Resonance Structures (cont.) The classic example: O 3. Both structures are correct!
- Slide 4
- Resonance Structures (cont.) In this example, O 3 has two resonance structures: Conceptually, we think of the bonding being an average of these two structures. Electrons are delocalized between the oxygens such that on average the bond strength is equivalent to 1.5 O-O bonds.
- Slide 5
- Structural Isomers What if different sets of atomic linkages can be used to construct correct LDSs: Both are correct, but which is more correct?
- Slide 6
- Formal Charge Formal Charge: Compare the nuclear charge (+Z) to the number of electrons (dividing bonding electron pairs by 2). Difference is known as the formal charge. #e- 7 6 7 7 6 7 Z + 7 6 7 7 7 6 Formal C. 0 0 0 0 +1 -1 Structure with less F. C. is more correct.
- Slide 7
- Formal Charge Example: CO 2 e - 6 4 6 6 4 67 4 5 Z + 6 4 6 6 6 46 6 4 FC 0 0 0 0 +2 -2-1 +2 -1 More Correct
- Slide 8
- Beyond the Octet Rule There are numerous exceptions to the octet rule. Well deal with three classes of violation here: Sub-octet systems Valence shell expansion Odd-electron systems
- Slide 9
- Beyond the Octet Rule (cont.) Some atoms (Be and B in particular) undergo bonding, but will form stable molecules that do not fulfill the octet rule. Experiments demonstrate that the B-F bond strength is consistent with single bonds only.
- Slide 10
- Beyond the Octet Rule (cont.) For third-row elements (Period 3), the energetic proximity of the d orbitals allows for the participation of these orbitals in bonding. When this occurs, more than 8 electrons can surround a third-row element. Example: ClF 3 (a 28 e - system) F obey octet rule Cl has 10e -
- Slide 11
- Beyond the Octet Rule (cont.) Finally, one can encounter odd electron systems where full pairs will not exist. Example: Chlorine Dioxide. Unpaired electron
- Slide 12
- Summary Remember the following: C, N, O, and F almost always obey the octet rule. B and Be are often sub-octet Second row (Period 2) elements never exceed the octet rule Third Row elements and beyond can use valence shell expansion to exceed the octet rule. In the end, you have to practice..a lot!
- Slide 13
- VESPR Background Recall from last lecture that we had two types of electron pairs: bonding and lone. The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding, but what about geometry? Valence Electron Shell Pair Repulsion (VESPR). 3D structure is determined by minimizing repulsion of electron pairs.
- Slide 14
- VESPR Background (cont.) Example: CH 4 Must consider both bonding and lone pairs in minimizing repulsion. Lewis Structure VESPR Structure
- Slide 15
- VESPR Background (cont.) Example: NH 3 (both bonding and lone pairs). Lewis Structure VESPR Structure
- Slide 16
- VESPR Applications The previous examples illustrate the strategy for applying VESPR to predict molecular structure: 1. Construct the Lewis Dot Structure 2. Arranging bonding/lone electron pairs in space such that repulsions are minimized.
- Slide 17
- VESPR Applications Linear Structures: angle between bonds is 180 Example: BeF 2 180
- Slide 18
- VESPR Applications Trigonal Planar Structures: angle between bonds is 120 Example: BF 3 120
- Slide 19
- VESPR Background (cont.) Pyramidal: Bond angles are

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