VSEPR Theory and Molecular Geometry

VSEPR THEORY AND MOLECULAR GEOMETRY

Mr. ChapmanChemistry 20

Back to the Basics It seems like we have known since the

beginning of our education that opposites attract and that particles of the same charge repel each other.

This simple fact is the basis for why molecules form in the shapes that they do, and it is called VSEPR theory. VSEPR stands for Valence Shell Electron Pair Repulsion Theory.

Molecular Shape Molecules have definite shapes and the shape of a

molecule controls some of its chemical and physical properties.

What is the difference between a chemical and physical property?

There are some important definitions we must understand in order to comprehend molecular geometry.

Definitions Bond lengths – the distances between the

nuclei of bonded atoms

Bond angles –is the angle formed between three atoms across at least two bonds.

Bond lengths and bond angles are determined by the pairs of electrons that are in the valence shell of the central atom.

Molecules Can Take on a Number of Different Shapes

Valence-Shell Electron-Pair Repulsion (VSEPR) Theory Electron pairs repel each other, whether they are

in chemical bonds (bond pairs) or unshared (lone pairs).

Electron pairs assume orientations about an atom to minimize repulsions.

In other words, electrons in the valence shell want to get as far away from each other as possible.

Important Point There is an important difference

between electron pair arrangement and molecular geometry, which we are going to see in the next few slides.

Electron pair arrangement determines the molecular geometry of the molecule.

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 atom

Arrangement 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 linearClass

# of atomsbonded to

central atom



MolecularGeometry

VSEPR

AB3 3 0 trigonal planar

trigonal planar

10.1


# of atomsbonded to

central atom



MolecularGeometry

VSEPR


trigonal planar

10.1

AB4 4 0 tetrahedral tetrahedral


# of atomsbonded to

central atom



MolecularGeometry

VSEPR


trigonal planar

10.1


AB5 5 0 trigonalbipyramidal

trigonalbipyramidal


# of atomsbonded to

central atom



MolecularGeometry

VSEPR


trigonal planar

10.1



trigonalbipyramidal

AB6 6 0 octahedraloctahedral

Class

# of atomsbonded to

central atom



MolecularGeometry

VSEPR


trigonal planar

AB2E 2 1 trigonal planar bent

10.1

Class

# of atomsbonded to

central atom



MolecularGeometry

VSEPR

AB3E 3 1


tetrahedral trigonal pyramidal

10.1

Class

# of atomsbonded to

central atom



MolecularGeometry

VSEPR


10.1

AB3E 3 1 tetrahedral trigonalpyramidal

AB2E2 2 2 tetrahedral bent

H

O

H

Class

# of atomsbonded to

central atom



MolecularGeometry

VSEPR

10.1


trigonalbipyramidal

AB4E 4 1 trigonalbipyramidal

distorted tetrahedron

Class

# of atomsbonded to

central atom



MolecularGeometry

VSEPR

10.1


trigonalbipyramidal



AB3E2 3 2 trigonalbipyramidal T-shaped

ClF

F

F

Class

# of atomsbonded to

central atom



MolecularGeometry

VSEPR

10.1


trigonalbipyramidal



AB3E2 3 2 trigonalbipyramidal T-shaped

AB2E3 2 3 trigonalbipyramidal linear

I

I

I

Class

# of atomsbonded to

central atom



MolecularGeometry

VSEPR

10.1


AB5E 5 1 octahedral square pyramidal

Br

F F

FF

F

Class

# of atomsbonded to

central atom



MolecularGeometry

VSEPR

10.1


AB5E 5 1 octahedral square pyramidal

AB4E2 4 2 octahedral square planar

Xe

F F

FF

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 SF4?

SO O

AB2E

bentS

F

F

F F

AB4E

distortedtetrahedron

10.1

Examples / Practice: Draw Lewis structures, build models, and predict

the shape of the following molecules:

1. Carbon tetrahydride2. Boron Trichloride3. Carbon dioxide (carbons and oxygens double

bonded to each other)4. Selenium dibromide5. Water6. Nitrogen trifluoride

VSEPR Theory and Molecular Geometry

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