CHAPTER 10 - MOLECULAR SHAPE AND THEORY OF BONDING



Chapter 9 – Covalent Bonding Theories

The Valence Shell Electron Pair Repulsion (VSEPR) Theory

The Basic Concept of VSEPR Model:

• In most molecules or polyatomic ions, valence electrons occur in pairs;

• Central atoms may contain bonding and nonbonding electron-pairs in valence shells;

• The geometrical orientation of electron-pairs around a central atom is such that the overall repulsions between these pairs are minimum;

• Repulsions between electron pairs are not the same; the repulsions are such that:

Lone-pair ( lone-pair > lone-pair ( bond-pair > bond-pair ( bond-pair;

(( means repulsion)

• Molecular shapes are determined by the relative orientation of bond-pair electrons around the central atom, but are also influenced by the presence of lone-pair electrons.

Orientations of Valence Electron pairs Around Central Atoms

No. of E-Pairs Geometry___________

2 linear

3 trigonal planar

4 tetrahedral

5 trigonal bipyramidal

6 octahedral

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Summary of Molecular Shapes predicted using the VSEPR Model

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Number of E-pair Number of Number of Molecular

E-pairs Geometry bond-pairs Lone-pairs Shapes Examples

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2 Linear 2 0 Linear BeCl2

3 trigonal planar 3 0 trigonal planar BF3

3 '' 2 1 V-shape GeCl2

4 tetrahedral 4 0 tetrahedral CH4

4 '' 3 1 trigonal pyramidal NH3

4 '' 2 2 V-shape H2O

5 trigonal 5 0 trigonal PCl5

bipyramidal bipyramidal

5 '' 4 1 seesaw SF4

5 '' 3 2 T-shape ClF3

5 '' 2 3 Linear XeF2

6 octahedral 6 0 octahedral SF6

6 '' 5 1 square pyramidal BrF5

6 '' 4 2 square planar XeF4

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Predicting Molecular Shapes Using The VSEPR Method

1. Write the correct Lewis structure for the molecule or polyatomic ion.

2. Determine the number of valence electron pairs and their geometrical orientation around the central atom that would yield the minimum repulsion.

3. If two or more nonbonding pairs occur, place them as far apart from each other in order to minimize repulsions.

4. Predict the molecular shape from the net orientation of the bonding pairs around the central atom.

Bond Angles

Bond angles in covalent molecules or polyatomic ions are determined by the number of electron pairs, their orientation (geometry), and the number of lone-pair electrons around the central atom. The following are some of the examples:

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Molecules No. of E-pair No. of Molecular Bond angles

E-pairs Geometry lone-pairs Shape

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BeCl2 2 linear 0 linear 180o

BF3 3 trigonal planar 0 trigonal planar 120o

GeCl2 3 trigonal planar 1 V-shape 120o

CH4 4 tetrahedral 0 tetrahedral 109.5o

NH3 4 tetrahedral 1 trigonal pyramid 107.3o

H2O 4 tetrahedral 2 V-shape 104.5o

PCl5 5 trigonal bipyramid 0 trigonal bipyramid 90o & 120o

SF4 5 trigonal bipyramid 1 see-saw ~90o & 120o

ClF3 5 trigonal bipyramid 2 T-shape ................
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