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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