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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 (which atoms connected to which, number of bonds),

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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 (which atoms connected to which, number of bonds), but what about the 3-d shapes, geometry? • Recall from last lecture that we had two types of electron pairs: bonding pairs and lone pairs • Valence Shell Electron Pair Repulsion (VSEPR): the 3-D structure is determined by minimizing repulsion of electron pairs.

  3. Key: Must consider both bonding and lone pairs in minimizing electron repulsion. Example: CH4 (bonding pairs only, no lone pairs) Lewis Structure VSEPR Structure

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

  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.

  6. Some useful shorthand notation: * Refer to central atom as “A” * Attached atoms are labelled “X” * Lone pairs are labelled “E” • Examples: H2O: AX2E2 CH4: AX4 BF3: AX3 PCl5: AX5 NH3: AX3E ClF3: AX3E2

  7. Case: Linear Structure (AX2): angle between bonds is 180° Example: BeF2 180°

  8. Case: Trigonal Planar Structure (AX3): The angle between bonds is 120° Example: BF3 120°

  9. Case: Pyramidal (AX3E): Bond angles are <120° structure is nonplanar due to repulsion of lone-pair. Example: NH3 107° Lewis VSEPR Structure Molecular shape

  10. Case: Tetrahedral (AX4): the angle between bonds is ~109.5° Example: CH4 109.5°

  11. Note: for ‘Tetrahedral’, the actual angle may vary slightly from 109.5°, due to size differences between bonding and lone pair electron densities bonding pair: more elongated, less repulsive lone pair: puffier, more repulsive

  12. Example of distorted tetrahedron: water (AX2E2): the angle is reduced to 104.5° by repulsion of the lone pairs “bent” Molecular shape VSEPR structure

  13. Compare: CH4 (AX4) NH3 (AX3E) H2O (AX2E2) Lone pairs: none one two

  14. Question? What is the approximate bond angle in SO2? A. 90° C. 120° D. 109.5° B. 180°

  15. Case: Trigonal Bipyramidal (AX5): non-equivalent bond positions: three in-plane (equatorial, 120°), and two at 90° to plane (axial) Example, PCl5 90° 120°

  16. Octahedral (AX6): all angles are 90°. Example SF6 90° Lewis VSEPR

  17. Special cases: See-Saw and Square Planar AX4E: ‘See Saw’ AX4E2: ‘Square Planar’ Note: error <120° 180° 90°

  18. Q? Which of these two possible arrangements is preferred? Square Planar is more stable, and non-polar, since the lone pairs can get farther apart (Would be polar) Obs: no dipole moment! Square Planar: not polar, due to symmetry

  19. • Driving force for last structure was to maximize the angular separation of the lone pairs. Same effect occurs in I3- (AX2E3): Less stable(stronger lone pair repulsion) Most stable

  20. What is the orientation of the ClF bonds in ClF3 (28 e-)? AX3E2 A B C Always put the lone pairs in the equatorial plane

  21. +1 +1 0 0 -1 -1 VSEPR and Resonance Structures Must look at VSEPR structures for all resonance species to predict molecular properties. Example: O3 (AX2E) bent Dipole moment? Yes!

  22. Give the Lewis dot and VSEPR structures for CF2Cl2. Does it have a dipole moment? (Yes) 32 e- Tetrahedral (AX4)

  23. What is the expected shape of ICl2+? AX2E2 A. linear C. tetrahedral D. square planar B. bent

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