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Lecture 25: VSEPR

Lecture 25: VSEPR. Reading: Zumdahl 13.13 Outline Concept behind VSEPR Molecular geometries. VSEPR Background. • The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding, but what about geometry?.

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Lecture 25: VSEPR

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

  2. VSEPR Background • The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding, but what about geometry? • Recall from last lecture that we had two types of electron pairs: bonding and lone. • Valence Shell Electron Pair Repulsion (VSEPR). 3D structure is determined by minimizing repulsion of electron pairs.

  3. VSEPR Background (cont.) • Must consider both bonding and lone pairs in minimizing repulsion. • Example: CH4 Lewis Structure VSEPR Structure

  4. VSEPR Background (cont.) • Example: NH3 (both bonding and lone pairs). Lewis Structure VSEPR Structure

  5. VSEPR Applications • The previous examples illustrate the strategy for applying VSEPR to predict molecular structure: 1. Construct the Lewis Dot Structure 2. Arranging bonding/lone electron pairs in space such that repulsion is minimized.

  6. VSEPR Applications • Linear Structures: angle between bonds is 180° • Example: BeF2 180°

  7. VSEPR Applications • Trigonal Planar Structures: angle between bonds is 120° • Example: BF3 120°

  8. VSEPR Background (cont.) • Pyramidal: Bond angles are <120°, and structure is nonplanar: • Example: NH3 107°

  9. VSEPR Applications • Tetrahedral: angle between bonds is ~109.5° • Example: CH4 109.5°

  10. VSEPR Applications • Tetrahedral: angle may vary from 109.5° exactly due to size differences between bonding and lone pair electron densities bonding pair lone pair

  11. VSEPR Applications • Classic example of tetrahedral angle shift from 109.5° is water:

  12. VSEPR Applications • Comparison of CH4, NH3, and H2O:

  13. VSEPR Applications • Trigonal Bipyramidal, 120° in plane, and two orbitals at 90° to plane: • Example, PCl5: 90° 120°

  14. VSEPR Applications • Octahedral: all angles are 90°: • Example, PCl6: 90°

  15. Advanced VSEPR Applications • Square Planar versus “See Saw” See Saw No dipole moment Square Planar

  16. Advanced VSEPR Applications • Driving force for last structure was to maximize the angular separation of the lone pairs.

  17. Advanced VSEPR Applications • VSEPR and resonance structures. Must look at VSEPR structures for all resonance species to predict molecular properties.

  18. VSEPR Applications • Provide the Lewis dot and VSEPR structures for CF2Cl2. Does it have a dipole moment? 32 e- Tetrahedral

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