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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 cont1
VSEPR Background (cont.)

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

Lewis Structure

VSEPR Structure

vsepr applications
VSEPR Applications

• The previous examples illustrate the stratgey for

applying VSEPR to predict molecular structure:

1. Construct the Lewis Dot Structure

2. Arranging bonding/lone electron pairs

in space such that repulsions are minimized.

vsepr applications cont
VSEPR Applications (cont.)

• Shorthand notation:

1. Refer to central atom as “A”

2. Attached atoms are referred to as “X”

3. Lone pairs are referred to as “E”

• Examples:

H2O: AX2E2 CH4: AX4

BF3: AX3 PCl5: AX5

NH3: AX3E ClF3: AX3E2

vsepr applications1
VSEPR Applications

• Linear Structures (AX2): angle between bonds is 180°

• Example: BeF2

180°

vsepr applications2
VSEPR Applications

• Trigonal Planar Structures (AX3): angle between bonds is 120°

• Example: BF3

120°

vsepr background cont2
VSEPR Background (cont.)

• Pyramidal (AX3E): Bond angles are <120°, and structure is nonplanar due to repulsion lone-pair.

• Example: NH3

107°

vsepr applications3
VSEPR Applications

• Tetrahedral (AX4): angle between bonds is ~109.5°

• Example: CH4

109.5°

vsepr applications4
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 applications5
VSEPR Applications

• Classic example of tetrahedral angle shift from 109.5° is water (AX2E2):

“bent”

vsepr applications6
VSEPR Applications

• Comparison of CH4 (AX4), NH3 (AX3E), and H2O (AX2E2):

slide14

What is the approximate bond angle in SO2?

A. 90°

C. 120°

D. 109.5°

B. 180°

vsepr applications7
VSEPR Applications

• Trigonal Bipyramidal (AX5), 120° in plane, and two orbitals at 90° to plane.

• Example, PCl5:

90°

120°

vsepr applications8
VSEPR Applications

• Octahedral (AX6): all angles are 90°.

• Example, SF6:

90°

advanced vsepr applications
Advanced VSEPR Applications

• See-Saw versus Square Planar

AX4E: See Saw

AX4E2: Square Planar

<120°

180°

90°

advanced vsepr applications1
Advanced VSEPR Applications

• Square Planar versus “See Saw”

See Saw

No dipole moment

Square Planar

advanced vsepr applications2
Advanced VSEPR Applications

• Driving force for last structure was to

maximize the angular separation of the lone pairs.

Same effect occurs in I3- (AX2E3):

advanced vsepr applications3

+1

+1

0

0

-1

-1

Advanced VSEPR Applications

• VSEPR and resonance structures. Must look at

VSEPR structures for all resonance species to

predict molecular properties. Example: O3 (AX2E):

bent

Dipole moment? Yes!

vsepr applications9
VSEPR Applications
  • Provide the Lewis dot and VSEPR structures for CF2Cl2. Does it have a dipole moment?

32 e-

Tetrahedral

slide23

What is the expected shape of ICl2+?

AX2E2

A. linear

C. tetrahedral

D. square planar

B. bent