Transcript Chapter 10 Chemical Bonding II

Chemistry: A Molecular Approach, 2nd Ed.
Nivaldo Tro
Chapter 10
Chemical
Bonding II
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Structure Determines Properties!
• Properties of molecular substances depend on
•
the structure of the molecule
The structure includes many factors, such as:
__________________________________
__________________________________
ionic, polar covalent, or covalent
__________________________________
• Bonding theory should allow you to predict the
shapes of molecules
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Molecular Geometry
• Molecules are 3-dimensional objects
• We often describe the shape of a molecule
•
•
with terms that relate to _______________
These geometric figures have characteristic
“corners” that indicate the positions of the
surrounding atoms around a central atom in
the center of the geometric figure
The geometric figures also have
characteristic angles that we call
_______________________________
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Lewis Theory Predicts
Electron Groups
• Lewis theory predicts there are regions of
•
•
electrons in an atom
Some regions result from placing shared pairs
of valence electrons between bonding nuclei
Other regions result from placing unshared
valence electrons on a single nuclei
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Using Lewis Theory to Predict
Molecular Shapes
• Lewis theory says that these regions of
electron groups should repel each other
 ______________________________________
• This idea can then be extended to predict the
shapes of molecules
 the position of atoms surrounding a central atom will be
determined by _______________________________
 the positions of the electron groups will be determined
by trying to __________________________________
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VSEPR Theory
• Electron groups around the central atom will
be most stable when they are as far apart as
possible – we call this
_____________________________________
_theory
 because electrons are negatively charged, they should be most
stable when they are separated as much as possible
• The resulting geometric arrangement will allow
us to predict the shapes and bond angles in
the molecule
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Electron Groups
• The Lewis structure predicts the number of valence
•
•
electron pairs around the central atom(s)
Each ________________of electrons constitutes
________________on a central atom
Each ___________constitutes _____________on
a central atom
 regardless of whether it is single, double, or triple
••
•O
•
••
N
••
O ••
••
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there are three electron groups on N
________________________
________________________
________________________
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Electron Group Geometry
• There are ___________________of electron
groups around a central atom
 based on a maximum of six bonding electron groups
 though there may be more than six on very large atoms, it is
very rare
• Each of these five basic arrangements results in
_____________ different basic electron
geometries
 in order for the molecular shape and bond angles to be a “perfect”
geometric figure, all the electron groups must be bonds and all the bonds
must be equivalent
• For molecules that exhibit _____________, it
doesn’t matter which resonance form you use – the
electron geometry will be the same
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Linear Electron Geometry
• When there are two electron groups around the
•
•
central atom, they will occupy positions on
opposite sides of the central atom
This results in the electron groups taking a
________________________ (add bond angle
to your vsepr chart!)
The bond angle is 180°
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Linear Geometry
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Trigonal Planar Electron Geometry
• When there are three electron groups around
•
•
the central atom, they will occupy positions in
the shape of a triangle around the central atom
This results in the electron groups taking a
____________________
The bond angle _________
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Trigonal Geometry
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Tetrahedral Electron Geometry
• When there are four electron groups around the
•
•
central atom, they will occupy positions in the
shape of a tetrahedron around the central atom
This results in the electron groups taking a
______________________
The bond angle is ____________°
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Tetrahedral Geometry
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Trigonal Bipyramidal Electron Geometry
• When there are five electron groups around the central
•
•
•
•
atom, they will occupy positions in the shape of two
tetrahedra that are base-to-base with the central atom in the
center of the shared bases
This results in the electron groups taking a
________________________________
The positions above and below the central atom are called
the ________________ positions
The positions in the same base plane as the central atom
are called the ______________equatorial positions is 120°
The bond angle between axial and equatorial positions is 90°
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Trigonal Bipyramid
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Trigonal Bipyramidal Geometry
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Octahedral Electron Geometry
• When there are six electron groups around the central
atom, they will occupy positions in the shape of two
square-base pyramids that are base-to-base with the
central atom in the center of the shared bases
• This results in the electron groups taking an
___________________________
 it is called octahedral because the geometric figure has
eight sides
• All positions are equivalent
• The bond angle ___________________°
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Octahedral Geometry
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Octahedral Geometry
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Molecular Geometry
• The actual geometry of the molecule may be
different from the electron geometry
• When the electron groups are attached to
atoms of different size, or when the bonding to
one atom is different than the bonding to
another, this will affect the molecular geometry
around the central atom
• _____________________________________
they occupy space on the central atom, but are not
“seen” as points on the molecular geometry
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Not Quite Perfect Geometry
Because the bonds and
atom sizes are not
identical in formaldehyde,
the observed angles are
slightly different from ideal
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The Effect of Lone Pairs
• Lone pair groups “occupy more space” on the
central atom
because their electron density is exclusively on the
central atom rather than shared like bonding electron
groups
• Relative sizes of repulsive force interactions is
Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair
• This affects the bond angles, making the bonding
pair – bonding pair angles smaller than expected
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Effect of Lone Pairs
bonding electrons
are are
shared
by twoon
The nonbonding
electrons
localized
atoms,
so some
negative
charge
is
the
central
atom,ofsothe
area
of negative
charge
removed
from
the central atom
takes
more
space
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Bond Angle Distortion
from Lone Pairs
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Bond Angle Distortion
from Lone Pairs
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Bent Molecular Geometry:
Derivative of Trigonal Planar Electron Geometry
• When there are three electron groups around
•
the central atom, and one of them is a lone pair,
the resulting shape of the molecule is called a
______________________________________
The bond angle is less than 120°
 because the lone pair takes up more space
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Pyramidal & Bent Molecular Geometries:
Derivatives of Tetrahedral Electron Geometry
• When there are four electron groups around the
central atom, and one is a lone pair, the result is
called a __________________________
 because it is a triangular-base pyramid with the
central atom at the apex
• When there are four electron groups around the
central atom, and two are lone pairs, the result is
called a ___________________________
 it is planar
 it looks similar to the trigonal planar—bent shape,
except the angles are smaller
• For both shapes, the bond angle is less than
________________.
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Methane
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Pyramidal Shape
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Pyramidal Shape
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Tetrahedral–Bent Shape
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Tetrahedral–Bent Shape
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Derivatives of the
Trigonal Bipyramidal Electron Geometry
• When there are five electron groups around the central atom,
•
and some are lone pairs, they will occupy the equatorial
positions because there is more room
When there are five electron groups around the central atom,
and one is a lone pair, the result is called the
__________________
 aka distorted tetrahedron
• When there are five electron groups around the central atom,
•
and two are lone pairs, the result is called the ____________
When there are five electron groups around the central atom,
and three are lone pairs, the result is a ________________
• The bond angles between equatorial positions are less than _______
• The bond angles between axial and equatorial positions are less than ___°
 linear = 180° axial–to–axial
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Replacing Atoms with Lone Pairs
in the Trigonal Bipyramid System
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Seesaw Shape
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T–Shape
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T–Shape
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Linear Shape
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Derivatives of the
Octahedral Geometry
• When there are six electron groups around the
•
central atom, and some are lone pairs, each even
number lone pair will take a position opposite the
previous lone pair
When there are six electron groups around the
central atom, and one is a lone pair, the result is
called a ___________________
 the bond angles between axial and equatorial positions is
less than 90°
• When there are six electron groups around the
central atom, and two are lone pairs, the result is
called a ____________________
 the bond angles between equatorial positions is 90°
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Square Pyramidal Shape
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Square Planar Shape
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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, ____________________________
4. Use Table 10.1 to determine the shape and
bond angles
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Example 10.2: Predict the geometry and bond
angles of PCl3
1. Draw the Lewis
structure
a) _____ valence
electrons
2. Determine the
Number of electron
groups around
central atom
a) ______ electron
groups around P
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Example 10.2: Predict the geometry and bond
angles of PCl3
3. Classify the electron groups
a) _______ bonding groups
b) _______ lone pair
4. Use Table 10.1 to determine the
shape and bond angles
a) four electron groups around P =
__________ electron geometry
b) three bonding + one lone pair =
____________molecular
geometry
c) trigonal pyramidal = bond angles
less than ________°
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Practice – Predict the molecular geometry
and bond angles in SiF5−
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Practice – Predict the molecular geometry
and bond angles in ClO2F
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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 ________________
For atoms in front of the plane, use a _________
For atoms behind the plane, use a ___________
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SF6
F
F
F
S
F
F
F
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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 _______________
shape around center C is ___________
shape around right O is _____________
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H
O


|
||
 
H  C  C  O  H
|
 
H
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Describing the Geometry
of Methanol
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Describing the Geometry
of Glycine
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Practice – Predict the molecular geometries
in H3BO3
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Polarity of Molecules
• For a molecule to be polar it must
1. have polar bonds


_______________________- theory
_________________ - measured
2. have an unsymmetrical shape

_________________________
• Polarity affects the intermolecular forces of
attraction
 therefore ______________and _____________

like dissolves like
• Nonbonding pairs affect molecular polarity,
strong pull in its direction
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Molecule Polarity
The H─Cl bond is polar. The bonding
electrons are pulled toward the Cl end of
the molecule. The net result is a polar
molecule.
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Molecule Polarity
The O─C bond ___________. The
bonding electrons are pulled equally
toward both O ends of the molecule. The
net result is a _____________ molecule.
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Molecule Polarity
The H─O bond is _________. Both
sets of bonding electrons are pulled
toward the O end of the molecule. The
net result is a _________ molecule.
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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
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Example 10.5: Predict whether NH3 is a
polar molecule
1. Draw the Lewis
structure and
determine the
molecular geometry
a) ___________ valence
electrons
b) three bonding + one lone
pair =
___________molecular
geometry
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Example 10.5: Predict whether NH3 is a
polar molecule
2. Determine if the bonds
are polar
a) ____________difference
b) if the bonds are not polar,
we can stop here and
declare the molecule will
be nonpolar
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ENN = 3.0
ENH = 2.1
3.0 − 2.1 =
____
therefore the
bonds are
___________
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Example 10.5: Predict whether NH3 is a
polar molecule
3) Determine whether
the polar bonds add
together to give a
net dipole moment
a) ______ addition
b) generally, asymmetric
shapes result in
uncompensated
polarities and a net
dipole moment
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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
molecule.
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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
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Molecular Polarity
Affects
Solubility in Water
• Polar molecules are
•
________to other polar
molecules
Because water is a _____
molecule, other _____
molecules dissolve well in
water
 and ionic compounds as well
• Some molecules have both
69
polar and nonpolar parts
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Orbital Diagram for the
Formation of H2S
H
1s
↑
+ ↑↓
1s
↑
3s
↑
↑ ↑↓ S
3p
↑↓
H─S bond
↑↓
H─S bond
H
Predicts bond angle = 90°
Actual bond angle = 92°
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Valence Bond Theory – Hybridization
• One of the issues that arises is that the number
of partially filled or empty atomic orbitals did
not predict the number of bonds or orientation
of bonds
C = 2s22px12py12pz0 would predict two or three
bonds that are 90° apart, rather than four bonds
that are 109.5° apart
• To adjust for these inconsistencies, it was
postulated that the valence atomic orbitals
could _____________ before bonding took
place
one hybridization of C is to mix all the 2s and 2p
orbitals to get four orbitals that point at the corners
of a tetrahedron
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Unhybridized C Orbitals Predict the
Wrong Bonding & Geometry
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Valence Bond Theory
Main Concepts
1. The valence electrons of the atoms in a
molecule reside in quantum-mechanical atomic
orbitals. The orbitals can be the standard s, p,
d, and f orbitals,
_______________combinations of these.
2. A chemical bond results when these atomic
orbitals interact and there is a total of two
electrons in the new molecular orbital
a) _____________________
3. The shape of the molecule is determined by the
geometry of the interacting orbitals
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Hybridization
• Some atoms hybridize their orbitals to
•
maximize bonding
• more bonds = more full orbitals = more stability
______________ is mixing different types of
orbitals in the valence shell to make a new set
of degenerate orbitals
sp, sp2, sp3, sp3d, sp3d2
• Same type of atom can have different types of
hybridization
C = sp, sp2, sp3
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Hybrid Orbitals
• The number of standard atomic orbitals
combined = the number of hybrid orbitals formed
combining a 2s with a 2p gives
____________orbitals
H cannot hybridize!!
its valence shell only has one orbital
• The number and type of standard atomic orbitals
•
combined determines ___________of the hybrid
orbitals
The particular kind of hybridization that occurs is
the one that yields the lowest overall energy for
the molecule
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Carbon Hybridizations
Unhybridized



2p
2s
sp hybridized



2sp

2p
sp2 hybridized

 
2sp2
sp3 hybridized




2p

2sp3
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sp3 Hybridization
• Atom with four electron groups around it
___________________etry
109.5° angles between hybrid orbitals
• Atom uses hybrid orbitals for all bonds and
lone pairs
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Orbital Diagram of the
sp3 Hybridization of C
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sp3 Hybridized Atoms
Orbital Diagrams
• Place electrons into hybrid and unhybridized valence orbitals
as if all the orbitals have equal energy
• Lone pairs generally occupy hybrid orbitals
sp3 hybridized atom
Unhybridized atom
2s

2s
 
2p
C

  
2p
N

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  
2sp3


  
2sp3
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Practice – Draw the orbital diagram for
the sp3 hybridization of each atom
Unhybridized atom
3s

2s
  
3p
Cl
   
3sp3
  
2p
O
   
2sp3
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

sp3 hybridized atom
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Bonding with Valence Bond Theory
• According to valence bond theory, bonding
takes place between atoms when their atomic
or hybrid orbitals interact
“overlap”
• To interact, the orbitals must either be aligned
•
along the axis between the atoms, or
The orbitals must be parallel to each other and
perpendicular to the interatomic axis
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Methane Formation with sp3 C
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Ammonia Formation with sp3 N
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Types of Bonds
• A _____________results when the interacting
atomic orbitals point along the axis connecting the
two bonding nuclei
 either standard atomic orbitals or hybrids
 s–to–s, p–to–p, hybrid–to–hybrid, s–to–hybrid, etc.
• A ______________results when the bonding atomic
orbitals are parallel to each other and perpendicular
to the axis connecting the two bonding nuclei
 between unhybridized parallel p orbitals
• The interaction between parallel orbitals is not as
strong as between orbitals that point at each other;
therefore _____________________________
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