Molecular Geometry• The properties of a compound are very much

determined by the size and shape of its molecules.

• Molecular geometry specifies the positions of the atoms in terms of bond lengths and bond angles.

• We will focus on the molecular shape which depends on bond angles.

Molecular ModelsMolecular geometry can be represented in different ways.

Perspective drawing:

Ball and Stick:

Space filling:

The VSEPR Model

• Valence Shell Electron Pair Repulsion is a simple but effective model for predicting molecular geometry.

• The first assumption of VSEPR is:

A molecule adopts the geometry that min- imizes the repulsive force among a given number of electron pairs.

Applying VSEPR

• Draw the Lewis structure of the molecule.

• Count the number of electron pairs around the central atom. Multiple bonds count as one electron pair.

• The arrangement of electron pairs that min- imizes repulsion is called the electron-pair geometry.

• The arrangement of atoms is called the molecular geometry.

Electron-Pair Geometries

#E.P.s Geometry

2 Linear

180

3 Trigonal Planar

120

4 Tetrahedral

109.5

Electron-Pair Geometries

#E.P.s Geometry

5 TrigonalBipyramidal

6 Octahedral

90

9090

120

QuickTime Movie

The Trigonal Bipyramid

There are two positions, axial and equatorial, and two bond angles - 90and 120.

Some Basic GeometriesMole- Lewis EP Bond cule Structure Geometry Angle

2 - Linear 180CO2

SO2 3 - Trigonal Planar

120

CO3-2 3 - Trigonal

Planar120

Raswin molecule

Raswin molecule

Raswin molecule

O C O= =

O S O=

O S O=

O C O

O

-2

Some Basic GeometriesMole- Lewis EP Bond cule Structure Geometry Angle

CH4 4 - Tetra-hedral

109.5

Raswin molecule

H C H

H

H

NH3 4 - Tetra-hedral

109.5

Raswin molecule

H N H

H

Some Basic GeometriesMole- Lewis EP Bond cule Structure Geometry Angle

SF4 5 -Trigonal Bipyra- midal

90,120

XeF4 6 - Octa-hedral

90

Raswin molecule

Raswin molecule

F S F

F

F

F Xe F

F

F

Distortions from Ideal GeometryFurther assumptions of VSEPR:

• Nonbonding EPs exert a greater repulsive force than bonding EPs.

• Multiple bonds exert a greater repulsive force than single bonds.

• Example: H2O OHH

104.5

Molecules With More than One Central AtomThe geometry around each central atom must be determined.

HC2H3O2:

Trigonal planar

Tetrahedral

H C C O H

H

H

O

Molecular Dipole Moments• A molecule is polar if its centers of positive

and negative charge are separated.

• Polarity causes a neutral molecule to interact with ions or other dipoles.

• Dipoles in an electric

field:

Molecular Dipole MomentsFor polyatomic molecules, the dipole moment is the geometric sum of all bond dipole moments.

CO2 - Nonpolar H2O - Polar

Molecular Dipole MomentsAre the following polar or nonpolar?

BF3: Trigonal Planar

Nonpolar

CH2O: Trigonal Planar

F B F

F

H C H

O

B

F

F F

120

C

O

H HPolar

120

Molecular Dipole MomentsAre the following polar or nonpolar?

CCl4: Tetrahedral

Nonpolar

CH3Cl: Tetrahedral

Polar

Cl C Cl

Cl

Cl

H C H

H

Cl

C

Cl

Cl ClCl

109

C

Cl

H HH