Download Molecular Geometry and Bonding Theories

Molecular Geometry
and Bonding Theories
Mr. Matthew Totaro
AP Chemistry
Legacy High School
© 2012 Pearson Education, 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.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, 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.
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
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.
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
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.”
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Electron-Domain
Geometries
Table 9.1 contains
the electron-domain
geometries for two
through six electron
domains around a
central atom.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, 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.
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
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.
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
Molecular Geometries
Within each electron domain, then, there might
be more than one molecular geometry.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, 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.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, 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.
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
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.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, 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.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, 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.
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
Trigonal Bipyramidal Electron
Domain
• There are two
distinct positions in
this geometry:
– Axial
– Equatorial
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Trigonal Bipyramidal Electron
Domain
Lower-energy conformations result from having
nonbonding electron pairs in equatorial, rather than
axial, positions in this geometry.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Trigonal Bipyramidal Electron
Domain
• There are four
distinct molecular
geometries in this
domain:
–
–
–
–
Trigonal bipyramidal
Seesaw
T-shaped
Linear
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Octahedral Electron Domain
• All positions are
equivalent in the
octahedral domain.
• There are three
molecular
geometries:
– Octahedral
– Square pyramidal
– Square planar
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Predicting the Shapes
Around Central Atoms
1. Draw the Lewis structure
2. Determine the number of electron groups
around the central atom
3. Classify each electron group as bonding or
lone pair, and count each type
–
remember, multiple bonds count as one group
4. Use cheat sheet to determine the shape and
bond angles
Molecular
Geometries
and Bonding
© 2012
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Example: Predict the geometry and bond angles of
PCl3
1. Draw the Lewis
structure
a) 26 valence
electrons
2. Determine the
Number of electron
groups around
central atom
a) four electron groups
around P
Tro: Chemistry: A Molecular Approach, 2/e
20
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
Example: Predict the geometry and bond angles of
PCl3
3. Classify the electron groups
a) three bonding groups
b) one lone pair
4. Use Table 10.1 to determine the
shape and bond angles
a) four electron groups around P =
tetrahedral electron geometry
b) three bonding + one lone pair =
trigonal pyramidal molecular
geometry
c) trigonal pyramidal = bond angles
less than 109.5°
Tro: Chemistry: A Molecular Approach, 2/e
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Molecular
Geometries
and Bonding
Practice – Predict the molecular geometry and bond
angles in SiF5−
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Practice – Predict the molecular geometry and bond
angles in SiF5─
Si least electronegative
5 electron groups on Si
Si is central atom
Si = 4e─
F5 = 5(7e─) = 35e─
(─) = 1e─
total = 40e─
5 bonding groups
0 lone pairs
Shape = trigonal bipyramid
Bond angles
Feq–Si–Feq = 120°
Feq–Si–Fax = 90°
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Practice – Predict the molecular geometry and bond
angles in ClO2F
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Practice – Predict the molecular geometry and bond
angles in ClO2F
Cl least electronegative
4 electron groups on Cl
Cl is central atom
Cl = 7e─
O2 = 2(6e─) = 12e─
F = 7e─
Total = 26e─
3 bonding groups
1 lone pair
Shape = trigonal pyramidal
Bond angles
O–Cl–O < 109.5°
O–Cl–F < 109.5°
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Representing 3-Dimensional Shapes on
a 2-Dimensional Surface
• One of the problems with drawing molecules is
trying to show their dimensionality
• By convention, the central atom is put in the
plane of the paper
• Put as many other atoms as possible in the same
plane and indicate with a straight line
• For atoms in front of the plane, use a solid
wedge
• For atoms behind the plane, use a hashed
Molecular
wedge
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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SF6
F
F
F
S
F
F
F
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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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.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Multiple Central Atoms
• Many molecules have larger structures with many
interior atoms
• We can think of them as having multiple central atoms
• When this occurs, we describe the shape around
each central atom in sequence
• •
shape around left C is tetrahedral
shape around center C is trigonal planar
shape around right O is tetrahedral-bent
H
O
•
•
|
||
• •
H − C − C − O − H
|
• •
H
Molecular
Geometries
and Bonding
© 2012
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Describing the Geometry
of Methanol
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
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Describing the Geometry
of Glycine
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Practice – Predict the molecular geometries in
H3BO3
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Practice – Predict the molecular geometries in
H3BO3
oxyacid, so H attached to O
34 electron
electron groups
groups on
on B
O
B least electronegative
O
B has
has
2
3 bonding groups
2
0 lone
ponepairs
pairs
B Is Central Atom
B = 3e─
O3 = 3(6e─) = 18e─
H3 = 3(1e─) = 3e─
Total = 24e─
Shape on B = trigonal planar
Shape on O = tetrahedral bent
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Molecular Polarity
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
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.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Polarity
By adding the
individual bond
dipoles, one can
determine the overall
dipole moment for the
molecule.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Polarity
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Molecule Polarity
The O─C bond is polar. The bonding
electrons are pulled equally toward both
O ends of the molecule. The net result is
a nonpolar molecule.
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Molecular
Geometries
and Bonding
Molecule Polarity
The H─O bond is polar. Both sets of
bonding electrons are pulled toward the
O end of the molecule. The net result
is a polar molecule.
© 2012
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Molecular
Geometries
and Bonding
Predicting Polarity of Molecules
1. Draw the Lewis structure and determine
the molecular geometry
2. Determine whether the bonds in the
molecule are polar
a) if there are not polar bonds, the molecule
is nonpolar
3. Determine whether the polar bonds add
together to give a net dipole moment
Molecular
Geometries
and Bonding
© 2012
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Example: Predict whether NH3 is a polar molecule
1. Draw the Lewis
structure and
determine the
molecular geometry
a) eight valence electrons
b) three bonding + one
lone pair = trigonal
pyramidal molecular
geometry
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
42 Pearson Education, Inc.
Molecular
Geometries
and Bonding
Example: Predict whether NH3 is a polar molecule
2. Determine if the bonds
are polar
a) electronegativity
difference
b) if the bonds are not
polar, we can stop here
and declare the
molecule will be
nonpolar
ENN = 3.0
ENH = 2.1
3.0 − 2.1 = 0.9
therefore the
bonds are
polar covalent
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
43 Pearson Education, Inc.
Example: Predict whether NH3 is a polar molecule
3) Determine whether
the polar bonds add
together to give a net
dipole moment
a) vector addition
b) generally, asymmetric
shapes result in
uncompensated
polarities and a net
dipole moment
The H─N bond is polar. All
the sets of bonding
electrons are pulled toward
the N end of the molecule.
The net result is a polar
Molecular
molecule.
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Practice – Decide whether the following molecules
are polar
EN
O = 3.5
N = 3.0
Cl = 3.0
S = 2.5
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
45 Pearson Education, Inc.
Practice – Decide whether the following molecules
are polar
Trigonal
Bent
Trigonal
Planar
2.5
1. polar bonds, N-O
2. asymmetrical shape
polar
1. polar bonds, all S-O
2. symmetrical shape
nonpolar
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
46 Pearson Education, Inc.
Molecular Polarity
Affects
Solubility in Water
• Polar molecules are attracted
to other polar molecules
• Because water is a polar
molecule, other polar molecules
dissolve well in water
– and ionic compounds as well
• Some molecules have both
polar and nonpolar parts
© 2012
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Molecular
Geometries
and Bonding
Orbital Hybridization
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Overlap and Bonding
• We think of covalent
bonds forming
through the sharing
of electrons by
adjacent atoms.
• In such an approach
this can only occur
when orbitals on the
two atoms overlap.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Overlap and Bonding
• Increased overlap brings
the electrons and nuclei
closer together while
simultaneously decreasing
electron– electron
repulsion.
• However, if atoms get too
close, the internuclear
repulsion greatly raises the
energy.
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
Hybrid Orbitals
• Consider beryllium:
– In its ground electronic
state, beryllium would not
be able to form bonds,
because it has no singly
occupied orbitals.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Hybrid Orbitals
But if it absorbs the
small amount of
energy needed to
promote an electron
from the 2s to the 2p
orbital, it can form
two bonds.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Hybrid Orbitals
• Mixing the s and p orbitals yields two degenerate
orbitals that are hybrids of the two orbitals.
– These sp hybrid orbitals have two lobes like a p orbital.
– One of the lobes is larger and more rounded, as is the s
orbital.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Hybrid Orbitals
• These two degenerate orbitals would align
themselves 180° from each other.
• This is consistent with the observed geometry of
beryllium compounds: linear.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Unhybridized C Orbitals Predict the
Wrong Bonding & Geometry
Molecular
Geometries
and Bonding
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Methane Formation with sp3 C
Molecular
Geometries
and Bonding
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Ammonia Formation with sp3 N
Molecular
Geometries
and Bonding
© 2012
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Example: Predict the hybridization and
bonding scheme for CH3CHO
Determine the
hybridization of the
interior atoms
C1 = tetrahedral
∴ C1 = sp3
C2 = trigonal planar
∴ C2 = sp2
Sketch the molecule and
orbitals
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Example: Predict the hybridization and
bonding scheme for CH3CHO
Label the bonds
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
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Practice – Predict the hybridization of
all the atoms in H3BO3
Molecular
Geometries
and Bonding
Tro: Chemistry: A Molecular Approach, 2/e
© 2012
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Practice – Predict the hybridization and bonding
scheme of all the atoms in NClO
••
•O
•
••
N
••
Cl ••
••
σ:Osp2─Nsp2
↑↓
↑↓
O ↑↓ N
↑↓
↑↓
Cl
↑↓
O
N
π:Op─Np
Tro: Chemistry: A Molecular Approach, 2/e
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σ:Nsp2─Clp
Cl
Molecular
Geometries
and Bonding
Valence Bond Theory
• Hybridization is a major player in this approach
to bonding.
• There are two ways orbitals can overlap to form
bonds between atoms.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Sigma (σ) Bonds
• Sigma bonds are characterized by
– Head-to-head overlap.
– Cylindrical symmetry of electron density about the
internuclear axis.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Pi (π) Bonds
• Pi bonds are characterized by
– Side-to-side overlap.
– Electron density above and below the internuclear
axis.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Single Bonds
Single bonds are always σ bonds, because σ
overlap is greater, resulting in a stronger bond and
more energy lowering.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Multiple Bonds
In a multiple bond, one of the bonds is a σ
bond and the rest are π bonds.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Multiple Bonds
• In a molecule like formaldehyde (shown at left),
an sp2 orbital on carbon overlaps in σ fashion
with the corresponding orbital on the oxygen.
• The unhybridized p orbitals overlap in π fashion.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Multiple Bonds
In triple bonds, as in
acetylene, two sp orbitals
form a σ bond between the
carbons, and two pairs of p
orbitals overlap in π fashion
to form the two π bonds.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Molecular
Geometries
and Bonding
© 2012
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Delocalized Electrons: Resonance
When writing Lewis structures for species
like the nitrate ion, we draw resonance
structures to more accurately reflect the
structure of the molecule or ion.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Delocalized Electrons: Resonance
• In reality, each of the four atoms
in the nitrate ion has a p orbital.
• The p orbitals on all three
oxygens overlap with the p
orbital on the central nitrogen.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Delocalized Electrons: Resonance
This means the π electrons are
not localized between the
nitrogen and one of the oxygens,
but rather are delocalized
throughout the ion.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Resonance
The organic molecule benzene has six σ bonds
and a p orbital on each carbon atom.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.
Resonance
• In reality the π electrons in benzene are not localized,
but delocalized.
• The even distribution of the π electrons in benzene
makes the molecule unusually stable.
Molecular
Geometries
and Bonding
© 2012 Pearson Education, Inc.