Molecular Geometry and Hybridization of Atomic

Orbitals

Molecular Geometry

Diatomic molecules are the easiest to visualize in three dimensions HCl Cl2

Diatomic molecules are linear

Valence Shell Electron Pair Repulsion Theory

• The ideal geometry of a molecule is determined by the way the electron pairs orient themselves in space

• The orientation of electron pairs arises from electron repulsions

• The electron pairs spread out so as to minimize repulsion

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VSEPR Theory Frequently, we will describe two geometries

for each molecule. 1. Electronic geometry is determined by the

locations of regions of high electron density around the central atom(s).

2. Molecular geometry determined by the arrangement of atoms around the central atom(s).

Electron pairs are not used in the molecular geometry determination just the positions of the atoms in the molecule are used.

The Valence Shell Electron Pair Repulsion model predicts shapes.

1. e- pairs stay as far apart as possible to minimize repulsions.

2. The shape of a molecule is governed by the number of bonds and lone pairs present.

3. Treat a multiple bond like a single bond when determining a shape. Each is a single e-group.

4. Lone pairs occupy more volume than bonds.

Predicting Molecular Shapes: VSEPR

Predicting Molecular Shapes

1. Draw Lewis structure2. Determine the number of electron

pairs around the central atom. Count a multiple bond as one pair.

3. Arrange electron pairs as shown in the next slide

Predicting Molecular Shapes:

Linear Triangular planar Tetrahedral

Triangular bipyramidal Octahedral

Basic shapes that minimize repulsions:

If the molecule contains:• only bonding pairs – the angles shown are correct.• lone pair/bond mixtures – the angles change a

little. lone pair/lone pair repulsions are largest. lone pair/bond pair are intermediate in strength. bond/bond interactions are the smallest.

linear triangular planar

tetrahedral triangular bipyramida

l

octahedral

Examples

Illustrate the geometry of the following molecules:

1. BeH2

2. CH4

3. BF3

4. PCl55. SF6

Molecular Geometry

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VSEPR Theory1 Lone pair to lone pair is the strongest repulsion.2 Lone pair to bonding pair is intermediate

repulsion.3 Bonding pair to bonding pair is weakest

repulsion. Mnemonic for repulsion strengths

lp/lp > lp/bp > bp/bp Lone pair to lone pair repulsion is why bond

angles in water are less than 109.5o.

Bond Angles and Lone Pairs

Ammonia and water show smaller bond angles than predicted from the ideal geometry The lone pair is larger in volume than a

bond pair There is a nucleus at only one end of the

bond so the electrons are free to spread out over a larger area of space

The A-X-E Notation

A denotes a central atom X denotes a terminal atom E denotes a lone pair Example

Water H2O

O is central Two lone pairs Two hydrogen

AX2E2

Multiple Bonds

Molecular Geometry Summary with Lone Pairs Included

The steps in determining a molecular shape.

Molecular formula

Lewis structure

Electron-group

arrangementBond

angles

Molecular shape

(AXmEn)

Count all e- groups around central atom (A)

Note lone pairs and double bonds

Count bonding and nonbonding

e- groups separately.

Step 1

Step 2

Step 3

Step 4

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Valence Bond (VB) Theory Covalent bonds are formed by the

overlap of atomic orbitals. Atomic orbitals on the central atom can

mix and exchange their character with other atoms in a molecule.

Process is called hybridization. Hybrids are common:

1. Pink flowers 2. Mules

Hybrid Orbitals have the same shapes as predicted by VSEPR.

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Valence Bond (VB) Theory

Regions of High Electron

Density

Electronic Geometry

Hybridization

2 Linear sp3 Trigonal

planarsp2

4 Tetrahedral sp3

5 Trigonal bipyramidal

sp3d

6 Octahedral sp3d2

Valence Bond Theory

Unpaired electrons from one atom pair with unpaired electrons from another atom and give rise to chemical bonds

Simple extension of orbital diagrams

Multiple Bonds

Hybrid Orbitals

Hybridization of the s and p orbitals on carbon. The four sp3 hybrid orbitals have equal energy. The four valence electrons are distributed evenly

across the sp3 hybrid orbitals. The angle between the sp3 hybrid orbitals is 109.5o.

Hybrid Orbitals

The number of hybrid orbitals obtained equals the number of atomic orbitals mixed.

The type of hybrid orbitals obtained varies with the types of atomic orbitals mixed.

Key Points

sp sp2 sp3 sp3d sp3d2

Types of Hybrid Orbitals

Figure 11.2 The sp hybrid orbitals in gaseous BeCl2.

atomic orbitals

hybrid orbitals

orbital box diagrams

Figure 11.3 The sp2 hybrid orbitals in BF3.

Figure 11.4 The sp3 hybrid orbitals in CH4.

Figure 11.5 The sp3 hybrid orbitals in NH3.

Figure 11.5 continued The sp3 hybrid orbitals in H2O.

Figure 11.6 The sp3d hybrid orbitals in PCl5.

Figure 11.7 The sp3d2 hybrid orbitals in SF6.

Hybrid Orbitals and Geometry