Molecular Geometry

1
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.

2
Molecular Models

• Molecular geometry can be represented in
  different ways.

Perspective drawing
Ball and Stick
Space filling
3
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.

4
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.

5
Electron-Pair Geometries

• E.P.s Geometry

180?
Linear
2
120?
Trigonal Planar
3
109.5?
4
Tetrahedral
6
Electron-Pair Geometries

• E.P.s Geometry

90?
Trigonal Bipyramidal
5
120?
90?
90?
6
Octahedral
7
The Trigonal Bipyramid

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

8
Some Basic Geometries

• Mole- Lewis EP
  Bond cule Structure
  Geometry Angle

2 -
Linear
180?
CO2
SO2
120?
3 -
Trigonal Planar
CO3-2
3 -
120?
Trigonal Planar
9
Some Basic Geometries

• Mole- Lewis EP
  Bond cule Structure
  Geometry Angle

109.5?
CH4
4 -
Tetra- hedral
NH3
4 -
Tetra- hedral
109.5?
10
Some Basic Geometries

• Mole- Lewis EP
  Bond cule Structure
  Geometry Angle

Trigonal Bipyra- midal
SF4
90?, 120?
5 -
Octa- hedral
90?
XeF4
6 -
11
Distortions from Ideal Geometry

• Further 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

12
Molecules With More than One Central Atom

• The geometry around each central atom must be
  determined.
• HC2H3O2

13
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

14
Molecular Dipole Moments

• For polyatomic molecules, the dipole moment is
  the geometric sum of all bond dipole moments.

CO2 - Nonpolar
H2O - Polar
15
Molecular Dipole Moments

• Are the following polar or nonpolar?

BF3
Trigonal Planar
Nonpolar
CH2O
Trigonal Planar
Polar
16
Molecular Dipole Moments

• Are the following polar or nonpolar?

Tetrahedral
CCl4
Nonpolar
CH3Cl
Tetrahedral
Polar