Molecular Geometry and Bonding Theories

AP Chemistry – Ch 9Mr. Christopherson

VSEPR Theory

Valence Shell Electron Pair Repulsion Theory

Electron pairs orient themselves around the central atom in order to minimize repulsive forces. They get as far apart from each other as possible.

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VSEPR Theory

Types of e- PairsBonding pairs - form bondsLone pairs - nonbonding electrons

Lone pairs repel

more strongly than

bonding pairs!!!

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VSEPR Theory

Lone pairs reduce the bond angle between atoms because they repel more strongly than bonding pairs.

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Draw the Lewis Diagram.

Count up e- pairs on central atom.double/triple bonds = ONE pair

Shape is determined by the # of bonding pairs and lone pairs.

Know the 8 common shapes & their bond angles!

Determining Molecular Shape

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Molecular Shapes

AB2

Linear AB3

Trigonal planar

AB4

Tetrahedral

AB3EAngular or Bent

AB3ETrigonal

pyramidal

AB2E2

Angular or Bent

......

The VSEPR Model

O OC

Linear

The Shapes of Some Simple ABn Molecules

O OS

BentO O

S

O

Trigonalplanar

FF

F

N

Trigonalpyramidal

Squareplanar

F F

Xe

F FF

F

FP

F

F

Trigonalbipyramidal

Octahedral

FF

F

S

F

F

F

SF6

SO2

Brown, LeMay, Bursten, Chemistry The Central Science, 2000, page 305

Bonding and Shape of Molecules

Number of Bonds

Number of LONE Pairs Shape Examples

2

3

4

3

2

0

0

0

1

2

Linear

Trigonal planar

Tetrahedral

Pyramidal

Bent

BeCl2

BF3

CH4, SiCl4

NH3, PCl3

H2O, H2S, SCl2

-Be-

B

C

N

:

O

:

:

CovalentStructure

Molecular Geometry

H

H

H

H

109.5o

C

Linear Trigonal planar

Tetrahedral

Trigonal pyramidalBent

109.5o

107.3o104.5o

H2O CH4 PCl3 BeH2 BF3 CO2

180o

molecular formula

structural formula

molecular shape

ball-and-stick model

CH4 C

H

H

HH

H

H

H

H

109.5o

C

tetrahedrontetrahedralshape ofmethane

CH

H

H

H

Methane & Carbon TetrachlorideMethane & Carbon Tetrachloridemolecular formula

structural formula

molecular shape

ball-and-stick model

CH4 C

H

H

HH

H

H

H

H

109.5o

C

CCl4

space-filling model

C

Cl

Cl

ClCl

C109.5o

H

HHH

N107o HH

H

..

O104.5o H

H

..

..

CH4, methane NH3, ammonia H2O, water

..

O

O

O

lone pairelectrons

OOO

O3, ozone

Common Molecular Shapes

2 total

2 bond

0 lone

LINEAR180°BeH2

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B BA

CO2

O C O2 total

2 bond

0 lone LINEAR

180°

Examples

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3 total

3 bond

0 lone

TRIGONAL PLANAR

120°

BF3

Common Molecular Shapes

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B

B

A

B

Common Molecular Shapes

3 total

2 bond

1 lone

BENT

<120°

SO2

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4 total

4 bond

0 lone

TETRAHEDRAL

109.5°

CH4

Common Molecular Shapes

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B

A

BB

B

4 total

3 bond

1 lone

TRIGONAL PYRAMIDAL

107°

NH3

Common Molecular Shapes

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PF3

4 total

3 bond

1 lone

TRIGONAL PYRAMIDAL

107°

F P FF

Examples

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4 total

2 bond

2 lone

BENT

104.5°

H2O

Common Molecular Shapes

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5 total

5 bond

0 lone

TRIGONAL BIPYRAMIDAL

120°/90°PCl5

Common Molecular Shapes

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A Be

Be

Be

Ba

Ba

6 total

6 bond

0 lone

OCTAHEDRAL

90°

SF6

Common Molecular Shapes

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B

B

B

B

B

BA