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Molecular Geometry and Molecular Geometry and VSEPR TheoryVSEPR Theory

Chapter 4 Pages 106-111 Chapter 4 Pages 106-111

Mrs. WestonMrs. Weston

Advanced ChemistryAdvanced Chemistry

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Molecular ShapesMolecular Shapes• The shape of a molecule plays a large part in

determining its properties and reactivities.• We can predict shapes by examining the

Lewis structure for orientation of electron pairs.

• Electron pairs arrange themselves to minimize electrical repulsion.

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VSEPR TheoryVSEPR Theory

In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion.

Electron pairs arrange themselves as far as possible from each other.

We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.

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Why is VSEPR Theory Important?Why is VSEPR Theory Important?

• Gives a specific shape due to the number of bonded and non-bonded electron pairs in a molecule

• Tells us the actual 3-D structure of a molecule

• Again in bonding, electron pairs want to be as far away from each other as possible.

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The VSEPR ModelThe VSEPR Model

Predicting Molecular GeometriesPredicting Molecular Geometries

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How does VSEPR THEORY work?How does VSEPR THEORY work?

We can use VSEPR theory using 4 steps.

1. Draw the Lewis Structure for the molecule.

Example: SiF4

F

F-Si-F

F

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How does VSEPR THEORY work?How does VSEPR THEORY work?

We can use VSEPR theory using 4 steps

1. Draw the Lewis Structure for the molecule.

2. Tally the number of bonding pairs and lone (non-bonding) pairs on the center atom. F

F-Si-F

F

Bonding pairs: 4

Lone pairs on central atom: 0

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How does VSEPR THEORY work?How does VSEPR THEORY work?

We can use VSEPR theory using 4 steps

1. Draw the Lewis Structure for the molecule

2. Tally the number of bonding pairs and lone pairs on the center atom.

3. Arrange the rest of the atoms so that they are as far away from each other as possible.

Si

F

FF F

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How does VSEPR THEORY work?How does VSEPR THEORY work?We can use VSEPR theory using 4 steps

1. Draw the Lewis Structure for the molecule

2. Tally the number of bonding pairs and lone pairs on the center atom. (Double and triple bonds only count as one pair.)

3. Arrange the rest of the atoms so that they are as far away from each other as possible

4. Give the type of geometry the molecule has: Tetrahedral

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The VSEPR ModelThe VSEPR Model

Difference between geometry and shapeDifference between geometry and shapeElectron Pair Geometry:We determine the geometry only looking at electrons.All the atoms that obey the octet rule and have single bonds have the same tetrahedral-like geometry.

Shape or Molecular Structure:We name the shape by the positions of atoms.We ignore lone pairs in the shape.

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VSEPR and Resulting ShapesVSEPR and Resulting Shapes

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The VSEPR ModelThe VSEPR Model

Predicting ShapePredicting Shape

Shape

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The VSEPR ModelThe VSEPR Model

Predicting ShapePredicting Shape

Shape

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The VSEPR ModelThe VSEPR ModelThe Effect of Nonbonding Electrons and The Effect of Nonbonding Electrons and Multiple Bonds on Bond AnglesMultiple Bonds on Bond AnglesBy experiment, the H-X-H bond angle decreases on moving from C to N to O:

Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs.Therefore, the bond angle decreases as the number of lone pairs increase.

OHH

104.5O107O

NHH

HC

H

HHH109.5O

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The VSEPR ModelThe VSEPR Model

The Effect of Nonbonding Electrons and The Effect of Nonbonding Electrons and Multiple Bonds on Bond AnglesMultiple Bonds on Bond AnglesSimilarly, electrons in multiple bonds repel more than electrons in single bonds.

C OCl

Cl111.4o

124.3o

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The VSEPR ModelThe VSEPR ModelMolecules with More than One Central AtomMolecules with More than One Central AtomIn acetic acid, CH3COOH, there are three central atoms.We assign the geometry about each central atom separately.

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Polarity of MoleculesPolarity of MoleculesA molecule is POLAR if its centers of positive and negative charges do not coincide.The molecule behaves as a dipole (Having two ends of opposite charge)

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Polarity of MoleculesPolarity of MoleculesDipole: def: two charges, equal in magnitude and opposite in sign, are separated by a distance

Polarity of Polyatomic MoleculesPolarity of Polyatomic Molecules•Each bond can be polar.•The orientation of these polar bonds determines whether the molecule is polar overall.•It is possible for a molecule with polar bonds to be either polar or non-polar.

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Polarity of MoleculesPolarity of MoleculesDipole Moments of Polyatomic MoleculesDipole Moments of Polyatomic MoleculesExample: in CO2, each C-O dipole is canceled because the molecule is linear. In H2O, the H-O dipoles do not cancel because the molecule is bent.

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Polarity of MoleculesPolarity of MoleculesDipole Moments of Polyatomic MoleculesDipole Moments of Polyatomic Molecules

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Hybrid OrbitalsHybrid Orbitals• s and p orbitals are used in bonding. It is

easy to tell which ones are used by looking at our molecule. (2s and 2p)

• For example, CH4. Looking again at the Lewis structure, we see that there are 4 bonds.

• The s orbital and three p orbitals can combine and form 4 sp3 bonds.

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Hybrid OrbitalsHybrid Orbitals

• Regions of electron density-EACH BOND AND LONE PAIR OF ELECTRONS ON THE CENTRAL ATOM IS KNOWN AS A REGION OF ELECTRON DENSITY.

• 4 regions of electron density-sp3 hybridized

• 3 regions of electron density-sp2 hybridized

• 2 regions of electron density-sp hybridized

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Hybrid OrbitalsHybrid Orbitals

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Hybrid OrbitalsHybrid OrbitalsSummarySummaryTo assign hybridization:

1. Draw a Lewis structure.2. Assign the electron pair geometry using VSEPR

theory.3. Use the electron pair geometry to determine the

hybridization.4. Name the shape by the positions of the atoms.