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Molecular Geometry and Polarity

Part I: Molecular Geometry – Valence Bond Theory (B)

Dr. Chin Chu

River Dell Regional High School

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Valence Bond Theory

Combines bonding with geometry.Addresses the equality of bonds in a molecule as far as length and energy

are concerned.

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Valence Bond Theory• Explains the structures of covalently bonded

molecules– ‘how’ bonding occurs

Principles of VB Theory:Bonds form from overlapping atomic orbitals and electron pairs are shared between two atoms

A new set of hybridized orbitals may form out of necessity.

Lone pairs of electrons are localized on one atom

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Hybridization• When atomic orbitals on the central atom do not

provide sufficient number of orbitals with single electron for proper bonding with surrounding atoms, mixing of atomic orbitals to produce hybrid orbitals with proper orientation in 3D space provides a viable solution.

• The mixing of several atomic orbitals to form the same number of hybrid orbitals.

• All the hybrid orbitals that form are the same in shape and energy but differing in orientation in 3D space.

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sp3 hybridization

One -s orbitals and 3 – p orbitals blended to give four identical

hybrid sp3 orbitals

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sp3 hybridization

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sp3 hybridization

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Geometry?

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Hybridization

• We blend the s and p orbitals of the valence electrons and end up with the tetrahedral geometry.

• We combine one s orbital and 3 p orbitals.

• sp3 hybridization has tetrahedral geometry.

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sp3 geometry

109.5º

• This leads to tetrahedral shape.

• Every molecule with a total of 4 atoms and lone pair is sp3 hybridized.

• Gives us trigonal pyramidal and bent shapes.

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sp2 Hybridization

We mix 1 – s orbital with 2- p orbitals to produce three identical

hybrid sp2 orbitals

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Examples? BF3, C2H4, AlCl3

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Where is the P orbital?

• Perpendicular

• The overlap of orbitals makes a sigma bond ( bond)

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CCH

H

H

H

sigma bonds in ethene

pi bonds in ethene

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Ethene, sp2 hybridization

Geometry?

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sp2 hybridization in C2H4

• trigonal planar

• 120º angle

• one bond

• One sigma and one pi bond between the C-C atoms

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sp Hybridization

We combine an s – orbital with a – p orbital to form two identical

hybrid sp orbitals

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Bonding in BeCl2: sp Orbital overlap

Lone pairs of Cl stay in the p- atomic orbitals

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What about sp

• one s and one p hybridize

• Linear

• C2H2

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sp hybridization

• end up with two lobes 180º apart.

• p orbitals are at right angles

• makes room for two bonds and two sigma bonds.

• a triple bond or two double bonds possible

Ethynesp hybridization + pi bonds

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sp3d Hybridization

Geometry?

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sp3d2 Hybridization

Geometry?

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# of Lone Pairs+

# of Bonded Atoms Hybridization Examples

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sp

sp2

sp3

sp3d

sp3d2

BeCl2

BF3

CH4, NH3, H2O

PCl5

SF6

To predict the hybridization of the central atom:

Count the number of lone pairs AND the numberof atoms bonded to the central atom. What is the GEOMETRY of electron pairs?

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Hybridization and Geometry of Electrons

• sp3: tetrahedral• sp2: trigonal planar• sp: linear• sp3d: trigonal bipyramid• sp3d2: octahedral• Geometry of molecule determined

by number of shared and unshared electrons.

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Types of Hybrid Orbitals: Summary

• sp3 : 1 s and 3 p orbitals mix to form 4 sp3 orbitals.

• sp2 :1 s and 2 p orbitals mix to form 3 sp2 orbitals leaving 1 p orbital intact.

• sp : s and 1 p orbitals mix to form 4 sp orbitals leaving 2 p orbitals intact.

• sp3d: five orbitals• sp3d2 : six orbitals