VSEPR Slides Reading: Shriver Chapter 2 Outline Concept behind - - PowerPoint PPT Presentation

VSEPR Slides

• Reading: Shriver Chapter 2
• Outline

– Concept behind VSEPR – Molecular geometries

VSEPR 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 Shell Electron Pair Repulsion (VSEPR).

3D structure is determined by minimizing repulsion of electron pairs.

VSEPR Background (cont.)

• Example: CH4
• Must consider both bonding and lone pairs in

minimizing repulsion.

H C H H H

Lewis Structure VSEPR Structure

VSEPR Background (cont.)

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

Lewis Structure VSEPR Structure

H N H H

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 Applications

• Linear Structures: angle between bonds is 180°

F Be F F Be F

• Example: BeF2

180°

VSEPR Applications

• Trigonal Planar Structures: angle between

bonds is 120°

• Example: BF3

F B F F F B F F

120°

VSEPR Background (cont.)

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

is nonplanar:

H N H H

• Example: NH3

107°

VSEPR Applications

• Tetrahedral: angle between bonds is ~109.5°
• Example: CH4

H C H H H

109.5°

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 Applications

• Classic example of tetrahedral angle shift from

109.5° is water:

VSEPR Applications

• Comparison of CH4, NH3, and H2O:

VSEPR Applications

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

P Cl Cl Cl Cl Cl

• Example, PCl5:

90° 120°

VSEPR Applications

• Octahedral: all angles are 90°:
• Example, PCl6:

90°

P Cl Cl Cl Cl Cl Cl

Advanced VSEPR Applications

• Square Planar versus “See Saw”

See Saw Square Planar

No dipole moment

Advanced VSEPR Applications

• Driving force for last structure was to

maximize the angular separation of the lone pairs.

Advanced VSEPR Applications

• VSEPR and resonance structures. Must look at

VSEPR structures for all resonance species to predict molecular properties.

O O O

O O O O

VSEPR Applications

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

C F F Cl Cl

32 e-

F F Cl Cl

Tetrahedral