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