VESPR Model and Geometry
Localized Electron ModelLocalized Electron Model
Lewis structures are an application of the “Localized Electron Model”Lewis structures are an application of the “Localized Electron Model”
L.E.M. says: Electron pairs can be thought of as “belonging” to pairs of atoms when bonding
Resonance points out a weakness in the Localized Electron Model.
ModelsModels Models are attempts to explain how nature Models are attempts to explain how nature
operates on the microscopic level based on operates on the microscopic level based on experiences in the macroscopic world.experiences in the macroscopic world.
Models can be physical as with this DNA model
Models can be mathematical
Models can be theoretical or philosophical
Fundamental Properties of ModelsFundamental Properties of ModelsA model does not equal reality.
Models are oversimplifications, and are therefore often wrong.
Models become more complicated as they age.
We must understand the underlying assumptions in a model so that we don’t misuse it.
VSEPR – Valence Shell Electron Pair RepulsionX + EX + E Overall StructureOverall Structure FormsForms
2 Linear AX2
3 Trigonal Planar AX3, AX2E
4 Tetrahedral AX4, AX3E, AX2E2
5 Trigonal bipyramidal AX5, AX4E, AX3E2, AX2E3
6 Octahedral AX6, AX5E, AX4E2
A = central atomA = central atom
X =X = atoms bonded to Aatoms bonded to AE = nonbonding electron pairs on AE = nonbonding electron pairs on A
VSEPR: Linear
AXAX22 COCO22
VSEPR: Trigonal Planar
AXAX33
AXAX22EE
BFBF33
SnClSnCl22
VSEPR: Tetrahedral
AXAX44
AXAX33EE
AXAX22EE22
CClCCl44
PClPCl33
ClCl22OO
VSEPR: Trigonal Bi-pyramidal
AXAX55
AXAX44EE
AXAX33EE22
AXAX22EE33
PClPCl55
SFSF44
ClFClF33
II33--
VSEPR: Octahedral
AXAX66
AXAX55EE
AXAX44EE22
SFSF66
IClICl44--
BrFBrF55
Actual shape
ElectronPairs Bonding
Pairs
Non-Bonding
Pairs
Shape
2 2 0 linear
3 3 0 trigonal planar
3 2 1 bent
4 4 0 tetrahedral
4 3 1 trigonal pyramidal
4 2 2 bent
Actual Shape
ElectronPairs Bonding
Pairs
Non-Bonding
Pairs
Shape
5 5 0 trigonal bipyrimidal
5 4 1 See-saw
5 3 2 T-shaped
5 2 3 linear
Actual ShapeElectronPairs Bonding
Pairs
Non-Bonding
PairsShape
6 6 0 Octahedral
6 5 1 Square Pyramidal
6 4 2 Square Planar
6 3 3 T-shaped
6 2 1 linear
Examples
• SiF4
• SeF4
• KrF4
• BF3
• PF3
• BrF3
No central atom
• Can predict the geometry of each angle.• build it piece by piece.
How well does it work?
• Does an outstanding job for such a simple model.
• Predictions are almost always accurate.• Like all simple models, it has exceptions.• Doesn’t deal with odd electrons
Polar molecules• Must have polar bonds• Must not be symmetrical• Symmetrical shapes include– Linear– Trigonal planar– Tetrahedral– Trigonal bipyrimidal– Octahedral– Square planar