Chapter 6
Section 3 Molecular Shapes
Chapter 6
Section 3 Molecular Shapes
Objectives
Determining Molecular Shapes
• Predict the shape of a molecule using VSEPR
theory.
• The three-dimensional shape of a molecule is
important in determining the molecule’s physical and
chemical properties.
• Associate the polarity of molecules with the shapes
of molecules, and relate the polarity and shape of
molecules to the properties of a substance.
A Lewis Structure Can Help Predict Molecular Shape
• You can predict the shape of a molecule by
examining the Lewis structure of the molecule.
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Determining Molecular Shapes, continued
Determining Molecular Shapes, continued
A Lewis Structure Can Help Predict Molecular Shape,
continued
• The valence shell electron pair repulsion (VSEPR)
theory is a theory that predicts some molecular
shapes based on the idea that pairs of valence
electrons surrounding an atom repel each other.
Electron Pairs Can Determine Molecular Shape
• According to the VSEPR theory, the shape of a
molecule is determined by the valence electrons
surrounding the central atom.
• Electron pairs are negative, so they repel each other.
• Therefore, the shared pairs that form different bonds
repel each other and remain as far apart as possible.
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Chapter 6
Section 3 Molecular Shapes
Determining Molecular Shapes, continued
Determining Molecular Shapes, continued
Electron Pairs Can Determine Molecular Shape,
continued
• For CO2, the two double bonds around the central
carbon atom repel each other and remain far apart.
Electron Pairs Can Determine Molecular Shape,
continued
• The four shared pairs of electrons in CH4 are farthest
apart when each pair is positioned at the corners of a
tetrahedron.
• For BF3, the three single bonds around the central
fluorine atom will be at a maximum distance apart.
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Chapter 6
Visual Concepts
Chapter 6
VSEPR and Basic Molecular Shapes
VSEPR and Lone Electron Pairs
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Visual Concepts
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Chapter 6
Section 3 Molecular Shapes
Predicting Molecular Shapes
Predicting Molecular Shapes
Sample Problem D
Sample Problem D Solution
Determine the shape of H2O.
Draw the Lewis structure for H2O.
Count the number of shared and unshared pairs of
electrons around the central atom.
H2O has two shared pairs and two unshared pairs.
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Chapter 6
Section 3 Molecular Shapes
Chapter 6
Section 3 Molecular Shapes
Molecular Shape Affects a Substance’s
Properties
Predicting Molecular Shapes
Sample Problem D Solution, continued
Find the shape that allows the shared and unshared
pairs of electrons to be as far apart as possible.
Shape Affects Polarity
• One property that shape determines is the polarity of
a molecule.
• The polarity of a molecule that has more than two
atoms depends on the polarity of each bond and the
way the bonds are arranged in space.
The water molecule will have a bent shape.
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Chapter 6
Section 3 Molecular Shapes
Molecular Shape Affects a Substance’s
Properties, continued
Chapter 6
Molecular Shape
Affects Polarity
Shape Affects Polarity, continued
• If two dipoles are arranged in opposite directions,
they will cancel each other.
• If two dipoles are arranged at an angle, they will not
cancel each other.
• Compare the molecules of nonpolar carbon dioxide,
CO2, which has a linear shape, and polar water,
H2O, which has a bent shape.
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