Molecular Geometry and Chemical Bonding Theory

1
Molecular Geometry
2
Molecular Structure

• Molecular geometry is the general shape of a
  molecule or the arrangement of atoms in three
  dimensional space.

Physical and chemical properties depend on the
geometry of a molecule.
3
3-D Molecular Structures
3-D Model
3-D Drawing
4
Does it matter?The Thalidomide Story
?
The chemical structure of thalidomide.
models enantiomers (mirror image)
The Same and Not the Same, by Roald
Hoffmann 1995, Columbia University Press
5
Does it matter?Fatty Acids
trans fatty acid
cis fatty acid
6
VSEPR Model

• The Valence Shell Electron Pair Repulsion model
  predicts the shapes of molecules and ions by
  assuming that the valence shell electron pairs
  are arranged as far from one another as possible
  to minimize the repulsion between them.

7
VSEPR Notation
AX3

• AXE shorthand notation
• A - central atom
• X - substituent atoms
• E - lone pair electrons

8
Predicting Molecular Geometry VSEPR

• Only five basic shapes.
• When a lone pair replaces an atom, the molecular
  geometry changes as well as the angles.
• Double and triple bonds are treated as single
  bonds.

9
Linear
bond lone Molecular pairs
pairs Geometry 2 0 linear
1 1-3 linear
10
Predicting Molecular Geometry
Example 1 BeCl2
AX2
1. Draw the Lewis structure
2. Two electron pairs around the central atom.
Two substituents Zero lone pairs.
Geometry is Linear. Bond angle is 180o.
11
Predicting Molecular Geometry
Example 2 HF
AX
1. Draw the Lewis structure
2. One electron pair between the two
atoms. Three lone pairs on fluorine.
Geometry is Linear.
12
Trigonal Planar
bond lone Molecular pairs
pairs Geometry
3 0 trigonal planar
Three e- pairs about central atom
13
Predicting Molecular Geometry
Example 3 BF3
..
AX3
Three electron pairs around the central atom.
Three substituents and Zero lone pairs.
trigonal planar
14
Tetrahedral
bond lone Molecular pairs
pairs Geometry
4 0 tetrahedral
Four e- pairs about central atom
15
Predicting Molecular Geometry

• CH4

Example 4
AX4
Four electron pairs around the central atom.
Four substituents and Zero lone pairs.
tetrahedral
16
Trigonal Pyramidal
bond lone Molecular pairs
pairs Geometry
3 1 trigonal
pyramidal
Four e- pairs about central atom
17
Predicting Molecular Geometry

• NH3

Example 5
AX4 AX3
Four electron pairs around the central atom.
Three substituents and One lone pair.
molecular geometry triangular pyramidal
18
Bent
bond lone Molecular pairs
pairs Geometry
2 2 bent
Four e- pairs about central atom
19
Predicting Molecular Geometry

• H2O

Example 6
AX4 AX2
Four electron pairs around the central atom.
Two substituents and Two lone pairs.
O H H
molecular geometry angular or bent
20
Predicting Molecular Geometry

• Tetrahedral - bond angles

Order of increasing repulsion bonding
pair-bonding pair lt bonding pair-lone pair lt
lone pair-lone pair