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VSEPR Theory

Valence Shell Electron Pair Repulsion Theory

Atomium Building,

Andre Waterkeyn (architect), 1958

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VSEPR Theory

  • A method for predicting the shape of a molecule from the knowledge of the groups of electrons around a central atom.
  • The shape of a molecule is very important for its physical and chemical properties.

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Demo: Water and a Balloon

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  • Electron pairs (bonding pairs as well as nonbonding pairs of electrons) repel one another.

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  • Result: the electron pairs move as far apart as possible around central atom to minimize repulsion.

VSEPR Theory

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Electron Domain: a region of electron density around a central atom

    • can be a bond (single, double, or triple)
    • can be a lone pair

Electron Domain

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

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​

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

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3 (AX3)

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

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3 (AX3)

Trigonal planer

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

​

3 (AX3)

Trigonal planer

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4 (AX4)

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​

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​

​

​

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

​

3 (AX3)

Trigonal planer

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4 (AX4)

Tetrahedral

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109.5o

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

​

3 (AX3)

Trigonal planer

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4 (AX4)

Tetrahedral

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5 (AX5)

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​

​

​

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

​

3 (AX3)

Trigonal planer

​

4 (AX4)

Tetrahedral

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5 (AX5)

Trigonal Bipyramidal

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​

​

​

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

​

3 (AX3)

Trigonal planer

​

4 (AX4)

Tetrahedral

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5 (AX5)

Trigonal Bipyramidal

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6 (AX6)

​

​

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Electron Domains (X):�Regions of Electrons Around a Central Atom (A)

Electron Domains

Arrangement

Diagram

2 (AX2)

Linear

​

3 (AX3)

Trigonal planer

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4 (AX4)

Tetrahedral

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5 (AX5)

Trigonal Bipyramidal

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6 (AX6)

Octahedral

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Lone Pairs

  • The shapes on the previous slides are the general arrangements for electron domains.
  • Whether the domain is a lone pair or a bonding pair affects the molecular geometry (name) of the shape of the molecule.
  • Lone pairs repel more than bonding pairs do and will decrease the bond angle slightly

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Molecular Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

2

(AX2)

2

0

Linear

​

180

Electron Domain Geometry: Linear

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

3

(AX3)

3

0

Trigonal Planer

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120

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​

​

​

​

​

Electron Domain Geometry: Trig Planar

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

3

(AX3)

3

0

Trigonal Planer

​

120

3

(AX2E)

2

1

Bent

​

<120

Electron Domain Geometry: Trig Planar

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

4

(AX4)

4

0

Tetra-hedral

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109.5

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Electron Domain Geometry: Tetrahedral

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

4

(AX4)

4

0

Tetra-hedral

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109.5

4

(AX3E)

3

1

Trigonal Pyrami-dal

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<109.5

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​

​

​

​

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Electron Domain Geometry: Tetrahedral

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

4

(AX4)

4

0

Tetra-hedral

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109.5

4

(AX3E)

3

1

Trigonal Pyrami-dal

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<109.5

4

(AX2E2)

2

2

Bent

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<109.5

Electron Domain Geometry: Tetrahedral

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

5

(AX5)

5

0

Trigonal Bipyramidal

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90 &

120

Electron Domain Geometry: Trig. Bipyr.

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

5

(AX5)

5

0

Trigonal Bipyramidal

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90 &

120

5

(AX4E)

4

1

See-saw

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<90 &

<120

5

(AX3E2)

3

2

T-shape

​

90

5

(AX2E3)

2

3

Linear

​

180

Electron Domain Geometry: Trig. Bipyr.

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

6

(AX6)

6

0

Octahedral

​

90

Electron Domain Geometry: Octahedral

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Geometries

# of Electron Domains

# of Bonded Atoms

# of Lone Pairs

Molecular Geometry

Name

Model

Approx. Bond Angle

6

(AX6)

6

0

Octahedral

​

90

6

(AX5E)

5

1

Square Pyramidal

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<90

6

(AX4E2)

4

2

Square Planer

​

90

6

(AX3E3)

3

3

T-Shape

​

<90

6

(AX2E4)

2

4

Linear

​

180

Electron Domain Geometry: Octahedral

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Seesaw

Electron Domain Geometry

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We’re stopping notes here for today�

Time to do a little practice on your own!

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Molecular Polarity

  • Molecules can be polar or nonpolar.
  • Molecular polarity depends on the symmetry of the molecule.
  • (Remember, polar means “having two sides”)

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Nonpolar Molecules

Nonpolar molecules are symmetrical and have dipoles that cancel out.

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Polar Molecules

Polar molecules are asymmetrical and have dipoles that do not cancel out. They therefore have an overall dipole moment.

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Practice

Determine whether each of the following molecules is polar or nonpolar.

Polar

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Practice

Determine whether each of the following molecules is polar or nonpolar.

Nonpolar

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Practice

Determine whether each of the following molecules is polar or nonpolar.

Nonpolar

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Practice

Determine whether each of the following molecules is polar or nonpolar.

Polar