VSEPR Valence Shell Electron Pair Repulsion

1
VSEPRValence Shell Electron Pair Repulsion

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

2
VSEPRValence Shell Electron Pair Repulsion

• Allows the prediction of molecular geometry
• An extension of the groups of electrons
  (effective electron pair) concept
• Involves sp and dsp types of hybridizations
• The central atom may or may not obey the octet
  rule.

3
To DetermineMolecular Geometry

• Count the number of valence electrons.
• Place a pair of electrons between the atoms bound
  together.
• Each non-central (terminal) atom is given
  electrons to fulfill its inert gas configuration.
• Any additional electron pairs become lone
  (unshared) electrons pairs on the central atom.
• The geometry is determined by the number of
  bonding and unshared pairs of electrons on the
  central atom.

4
Consider the Possible Combinations
Atoms Bound to the Central Atom
1
2
3
4
5
6
0
1
Number of Lone Electron Pairs
2
3
What is the preferred geometry for the blocks
shown in blue?
5
Zero Lone Pairs of ElectronsAround the Central
Atom
Arrangement
Geometry
Example
6
Zero Lone Pairs of ElectronsAround the Central
Atom
Arrangement
Geometry
Example
7
Zero Lone Pairs of ElectronsAround the Central
Atom
8
AB Type Moleculeswith Unshared Pairs of Electrons
Arrangement
Geometry
Example
9
Molecular Models
Look at the various molecular models The blue
sphere represents an atom The paddle represents
a pair of electrons
Only atoms can be seen.
What is the shape of each molecule?
10
AB2 Type Moleculeswith Unshared Pairs of
Electrons
Arrangement
Geometry
Example
11
AB3 Type Moleculeswith Unshared Pairs of
Electrons
12
AB4 Type Moleculeswith Unshared Pairs of
Electrons
13
AB5 Type Moleculeswith Unshared Pairs of
Electrons
14
Lone Electron Pair Position

• The lone pairs always occur in the equatorial
  plane.
• Maximizes the angular separation between lone
  pairs and other electron pairs.

15
Summary of Observed Geometries
16
(No Transcript)
17
(No Transcript)
18
Skeleton Diagrams of Molecules
_at_º
19
Skeleton Diagrams of Molecules