Transcript Chapter 9 Molecular Geometries and Bonding Theories

Chemistry, The Central Science, 11th edition
Theodore L. Brown, H. Eugene LeMay, Jr.,
and Bruce E. Bursten
Chapter 9
Molecular Geometries
and Bonding Theories
John D. Bookstaver
St. Charles Community College
Cottleville, MO
© 2009, Prentice-Hall, Inc.
Molecular Shapes
• The shape of a molecule
plays an important role
in its reactivity.
• By noting the number of
bonding and nonbonding
electron pairs we can
easily predict the shape
of the molecule.
© 2009, Prentice-Hall, Inc.
What Determines the Shape of a
Molecule?
• Simply put, electron pairs,
whether they be bonding
or nonbonding, repel each
other.
• By assuming the electron
pairs are placed as far as
possible from each other,
we can predict the shape of
the molecule.
© 2009, Prentice-Hall, Inc.
Electron Domains
• The central atom in
this molecule, A, has
four electron
domains.
• We can refer to the
electron pairs as electron
domains.
• In a double or triple bond,
all electrons shared
between those two atoms
are on the same side of the
central atom; therefore,
they count as one electron
domain.
© 2009, Prentice-Hall, Inc.
Valence Shell Electron Pair Repulsion
Theory (VSEPR)
“The best
arrangement of a
given number of
electron domains is
the one that minimizes
the repulsions among
them.”
© 2009, Prentice-Hall, Inc.
Electron-Domain
Geometries
These are the
electron-domain
geometries for two
through six electron
domains around a
central atom.
© 2009, Prentice-Hall, Inc.
Electron-Domain Geometries
• All one must do is
count the number of
electron domains in
the Lewis structure.
• The geometry will be
that which
corresponds to the
number of electron
domains.
© 2009, Prentice-Hall, Inc.
Molecular Geometries
• The electron-domain geometry is often not the
shape of the molecule, however.
• The molecular geometry is that defined by the
positions of only the atoms in the molecules, not
the nonbonding pairs.
© 2009, Prentice-Hall, Inc.
Molecular Geometries
Within each electron
domain, then, there
might be more than
one molecular
geometry.
© 2009, Prentice-Hall, Inc.
Linear Electron Domain
• In the linear domain, there is only one molecular
geometry: linear.
• NOTE: If there are only two atoms in the
molecule, the molecule will be linear no matter
what the electron domain is.
© 2009, Prentice-Hall, Inc.
Trigonal Planar Electron Domain
• There are two molecular geometries:
– Trigonal planar, if all the electron domains are bonding,
– Bent, if one of the domains is a nonbonding pair.
© 2009, Prentice-Hall, Inc.
Nonbonding Pairs and Bond Angle
• Nonbonding pairs are physically
larger than bonding pairs.
• Therefore, their repulsions are
greater; this tends to decrease bond
angles in a molecule.
© 2009, Prentice-Hall, Inc.
Multiple Bonds and Bond Angles
• Double and triple
bonds place greater
electron density on
one side of the central
atom than do single
bonds.
• Therefore, they also
affect bond angles.
© 2009, Prentice-Hall, Inc.
Tetrahedral Electron Domain
• There are three molecular geometries:
– Tetrahedral, if all are bonding pairs,
– Trigonal pyramidal if one is a nonbonding pair,
– Bent if there are two nonbonding pairs.
© 2009, Prentice-Hall, Inc.
Trigonal Bipyramidal Electron Domain
• There are two distinct
positions in this
geometry:
– Axial
– Equatorial
© 2009, Prentice-Hall, Inc.
Trigonal Bipyramidal Electron Domain
Lower-energy conformations result from having
nonbonding electron pairs in equatorial, rather
than axial, positions in this geometry.
© 2009, Prentice-Hall, Inc.
Trigonal Bipyramidal Electron Domain
• There are four distinct
molecular geometries
in this domain:
–
–
–
–
Trigonal bipyramidal
Seesaw
T-shaped
Linear
© 2009, Prentice-Hall, Inc.
Octahedral Electron Domain
• All positions are
equivalent in the
octahedral domain.
• There are three
molecular
geometries:
– Octahedral
– Square pyramidal
– Square planar
© 2009, Prentice-Hall, Inc.
Larger Molecules
In larger molecules, it
makes more sense to
talk about the
geometry about a
particular atom rather
than the geometry of
the molecule as a
whole.
© 2009, Prentice-Hall, Inc.
Larger Molecules
This approach makes
sense, especially
because larger
molecules tend to
react at a particular
site in the molecule.
© 2009, Prentice-Hall, Inc.
Sample Exercise 9.1 Using the VSEPR Model
Use the VSEPR model to predict the molecular geometry of (a) O3, (b) SnCl3–.
Solution
1. Draw Lewis structure
2. Count the number of electron domains around the central atom (includes
nonbonding pairs, single, double, and triple bonds
3. Arrange the domains of electrons as far away in 3-D space as possible
(minimize repulsions). And determine molecular geometry.
Sample Exercise 9.1 Using the VSEPR Model
Solution (continued)
1. Draw Lewis structure
2. Count the number of electron domains around the central atom (includes
nonbonding pairs, single, double, and triple bonds
3. Arrange the domains of electrons as far away in 3-D space as possible (minimize
repulsions). And determine molecular geometry.
Practice Exercise
Predict the electron-domain geometry and the molecular geometry for (a) SeCl2, (b)
CO32–.
Answer: (a) tetrahedral, bent; (b) trigonal planar, trigonal planar
Sample Exercise 9.2 Molecular Geometries of Molecules with Expanded Valance
Shells
Use the VSEPR model to predict the molecular geometry of (a) SF4, (b) IF5.
Solution
Seesaw shaped
(distorted
tetrahedron)
Comment: The experimentally observed structure is shown on the right. We can
infer that the nonbonding electron domain occupies an equatorial position, as
predicted. The axial and equatorial S—F bonds are slightly bent back away from
the nonbonding domain, suggesting that the bonding domains are “pushed” by
the nonbonding domain, which is larger and has greater repulsion (Figure 9.7).
Sample Exercise 9.2 Molecular Geometries of Molecules with Expanded Valance
Shells
Solution (continued)
Square pyramidal (Table 9.3):
Comment: Because the domain for the nonbonding pair is larger than the other
domains, the four F atoms in the base of the pyramid are tipped up slightly toward
the F atom on top. Experimentally, we find that the angle between the base and
top F atoms is 82°, smaller than the ideal 90°angle of an octahedron.
Practice Exercise
Predict the electron-domain geometry and molecular geometry of (a) ClF3, (b) ICl4–.
Answer: (a) trigonal bipyramidal, T-shaped; (b) octahedral, square planar
Sample Exercise 9.3 Predicting Bond Angles
Practice Exercise
Predict the H—C—H and C—C—C bond angles in the following molecule, called
propyne:
Answer: 109.5°, 180°
Polarity
• In Chapter 8 we discussed
bond dipoles.
• But just because a
molecule possesses polar
bonds does not mean the
molecule as a whole will
be polar.
© 2009, Prentice-Hall, Inc.
Polarity
By adding the
individual bond
dipoles, one can
determine the overall
dipole moment for the
molecule.
© 2009, Prentice-Hall, Inc.
Polarity
© 2009, Prentice-Hall, Inc.
Sample Exercise 9.4 Polarity of Molecules
Predict whether the following molecules are polar or nonpolar: (a) BrCl, (b)
SO2, (c) SF6.
Solution
Sample Exercise 9.4 Polarity of Molecules
Solution (continued)
Practice Exercise
Determine whether the following molecules are polar or nonpolar: (a) NF3, (b)
BCl3.
Answer: (a) polar because polar bonds are arranged in a trigonal-pyramidal
geometry, (b) nonpolar because polar bonds are arranged in a trigonal-planar
geometry