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

vsepr background cont
VSEPR Background (cont.)

• Must consider both bonding and lone pairs in

minimizing repulsion.

  • Example: CH4

Lewis Structure

VSEPR Structure

vsepr background cont4
VSEPR Background (cont.)

• Example: NH3 (both bonding and lone pairs).

Lewis Structure

VSEPR Structure

vsepr applications
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.

vsepr applications6
VSEPR Applications

• Linear Structures: angle between bonds is 180°

• Example: BeF2

180°

vsepr applications7
VSEPR Applications

• Trigonal Planar Structures: angle between bonds is 120°

• Example: BF3

120°

vsepr background cont8
VSEPR Background (cont.)

• Pyramidal: Bond angles are <120°, and structure

is nonplanar:

• Example: NH3

107°

vsepr applications9
VSEPR Applications

• Tetrahedral: angle between bonds is ~109.5°

• Example: CH4

109.5°

vsepr applications10
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

vsepr applications11
VSEPR Applications

• Classic example of tetrahedral angle shift from 109.5° is water:

vsepr applications12
VSEPR Applications

• Comparison of CH4, NH3, and H2O:

vsepr applications13
VSEPR Applications

• Trigonal Bipyramidal, 120° in plane, and two orbitals at 90° to plane:

• Example, PCl5:

90°

120°

vsepr applications14
VSEPR Applications

• Octahedral: all angles are 90°:

• Example, PCl6:

90°

advanced vsepr applications
Advanced VSEPR Applications

• Square Planar versus “See Saw”

See Saw

No dipole moment

Square Planar

advanced vsepr applications16
Advanced VSEPR Applications

• Driving force for last structure was to

maximize the angular separation of the lone pairs.

advanced vsepr applications17
Advanced VSEPR Applications

• VSEPR and resonance structures. Must look at

VSEPR structures for all resonance species to

predict molecular properties.

vsepr applications18
VSEPR Applications
  • Provide the Lewis dot and VSEPR structures for CF2Cl2. Does it have a dipole moment?

32 e-

Tetrahedral