Molecular Geometry and VSEPR Theory

VSEPR Theory

• Valence Shell Electron Pair Repulsion Theory

• States that electron pairs repel each other and assume an orientation about an atom to minimize repulsion – the shape a molecule takes is due to this repulsion

Valence shell electron pair repulsion (VSEPR) model:

Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs.

AB2 2 0

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement of electron pairs

MolecularGeometry

10.1

linear linear

B B

Cl ClBe

2 atoms bonded to central atom0 lone pairs on central atom 10.1

AB2 2 0 linear linear

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement of electron pairs

MolecularGeometry

VSEPR

AB3 3 0trigonal planar

trigonal planar

10.1

10.1

AB2 2 0 linear linear

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement of electron pairs

MolecularGeometry

VSEPR

AB3 3 0trigonal planar

trigonal planar

10.1

AB4 4 0 tetrahedral tetrahedral

10.1

AB2 2 0 linear linear

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement of electron pairs

MolecularGeometry

VSEPR

AB3 3 0trigonal planar

trigonal planar

10.1

AB4 4 0 tetrahedral tetrahedral

AB5 5 0trigonal

bipyramidaltrigonal

bipyramidal

10.1

AB2 2 0 linear linear

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement of electron pairs

MolecularGeometry

VSEPR

AB3 3 0trigonal planar

trigonal planar

10.1

AB4 4 0 tetrahedral tetrahedral

AB5 5 0trigonal

bipyramidaltrigonal

bipyramidal

AB6 6 0 octahedraloctahedral

10.1

10.1

bonding-pair vs. bondingpair repulsion

lone-pair vs. lone pairrepulsion

lone-pair vs. bondingpair repulsion> >

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement of electron pairs

MolecularGeometry

VSEPR

AB3 3 0trigonal planar

trigonal planar

AB2E 2 1trigonal planar

bent

10.1

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement of electron pairs

MolecularGeometry

VSEPR

AB3E 3 1

AB4 4 0 tetrahedral tetrahedral

tetrahedraltrigonal

pyramidal

10.1

Class

# of atomsbonded to

central atom

# lonepairs on

central atomArrangement of electron pairs

MolecularGeometry

VSEPR

AB4 4 0 tetrahedral tetrahedral

10.1

AB3E 3 1 tetrahedraltrigonal

pyramidal

AB2E2 2 2 tetrahedral bent

H

O

H

10.1

Predicting Molecular Geometry1. Draw Lewis structure for molecule.

2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom.

3. Use VSEPR to predict the geometry of the molecule.

What are the molecular geometries of SO2 and CCl4?(In SF4, S uses an expanded octet of 10.)

AB2E bent

C

Cl

Cl

Cl Cl

AB4

Tetrahedral

10.1

SO

OS

OO

Parent shapes for EXn molecules (n = 2-5)

Formula n shape shapes of structures

EX2 2 linear

EX3 3 trigonal planar

EX4 4 tetrahedral

EX5 5 trigonal

bipyramidal

Parent shapes for EXn molecules (n = 6-8)

Formula n shape shapes of structures

EX6 6 octahedral

EX7 7 pentagonal

bipyramidal

EX8 8 square

antiprismatic

Activity• Go around to each lab station• At each station there are two molecules• In your data table,

– write the chemical formula (determine the class)– draw the Lewis structure– determine the molecular geometry

• Example:

Compound Lewis Structure Molecular Geometry

NH3

AB3E

Trigonal

Bipyramidal

ActivityCompound Lewis Structure Molecular

Geometry

CO2

AB2

Linear

CH2O

AB3

Trigonal

Planer

NO2-

AB2EBent

CH4

AB4

Tetrahedral

ActivityCompound Lewis Structure Molecular

Geometry

NH3

AB3E

Trigonal

Pyramidal

H2O

AB2E2

Bent

PCl5

AB5

Trigonal

Bipyramidal

SI6

AB6

Octohedral