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Lecture 25: VSEPR • Reading: Zumdahl 13.13 • Outline – Concept behind VSEPR – Molecular geometries
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Lecture 25: VSEPR Reading: Zumdahl 13.13 Outline –Concept behind VSEPR –Molecular geometries.

Dec 16, 2015

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  • Slide 1
  • Lecture 25: VSEPR Reading: Zumdahl 13.13 Outline Concept behind VSEPR Molecular geometries
  • Slide 2
  • VSEPR Background Recall from last lecture that we had two types of electron pairs: bonding pairs and lone pairs The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding (which atoms connected to which, number of bonds), but what about the 3-d shapes, geometry? Valence Shell Electron Pair Repulsion (VSEPR): the3-D structure is determined by minimizing repulsion of electron pairs.
  • Slide 3
  • Example: CH 4 (bonding pairs only, no lone pairs) Key: Must consider both bonding and lone pairs in minimizing electron repulsion. Lewis Structure VSEPR Structure
  • Slide 4
  • Example: NH 3 (both bonding and lone pairs). Lewis Structure VSEPR Structure Molecular Shape
  • Slide 5
  • VSEPR Applications The previous examples illustrate the stratgey for applying VSEPR to predict molecular structure: 1. Construct the Lewis Dot Structure 2. Arrange the bonding and lone electron pairs in space such that repulsions are minimized.
  • Slide 6
  • Some useful shorthand notation: * Refer to central atom as A * Attached atoms are labelled X * Lone pairs are labelled E Examples: H 2 O: AX 2 E 2 CH 4 : AX 4 BF 3 : AX 3 PCl 5 : AX 5 NH 3 : AX 3 E ClF 3 : AX 3 E 2
  • Slide 7
  • Case: Linear Structure (AX 2 ): angle between bonds is 180 Example: BeF 2 180
  • Slide 8
  • Case: Trigonal Planar Structure (AX 3 ): The angle between bonds is 120 Example: BF 3 120
  • Slide 9
  • Case: Pyramidal (AX 3 E): Bond angles are