Download F - HCC Learning Web

Chemical Bonding II:
Molecular Geometry and
Hybridization of Atomic Orbitals
Chapter 10
1
Molecular Geometry
and Bond Theory
• In this chapter we will discuss the geometries of
molecules in terms of their electronic structure –.
We will also explore two theories of chemical
bonding: valence bond theory and molecular
orbital theory.
Molecular geometry is the general shape of a
molecule, as determined by the relative
positions of the atomic nuclei- three
dimensional arrangement of atoms in a
molecule
The Valence-Shell Electron Pair
Repulsion Model
• The valence-shell electron pair repulsion
(VSEPR) model predicts the shapes of
molecules and ions by assuming that the
valence shell electron pairs are arranged as far
from one another as possible.
To predict the relative positions of atoms
around a given atom using the VSEPR
model, you first note the arrangement of the
electron pairs around that central atom.
Predicting Molecular Geometry
• The following rules and figures will help
discern electron pair arrangements.
1. Draw the Lewis structure
2. Determine how many electrons pairs are
around the central atom. Count a multiple
bond as one pair.
3. Arrange the electrons pairs.
Arrangement of Electron Pairs
About an Atom
2 pairs
Linear
3 pairs
Trigonal planar
5 pairs
Trigonal bipyramidal
4 pairs
Tetrahedral
6 pairs
Octahedral
Valence shell electron pair repulsion (VSEPR) model:
Predict the geometry of the molecule from the electrostatic
repulsions between the electron (bonding and nonbonding) pairs.
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
B
B
0 lone pairs on central atom
Cl
Be
Cl
2 atoms bonded to central atom
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
0
trigonal
planar
trigonal
planar
AB3
3
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
trigonal
planar
tetrahedral
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
trigonal
planar
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
AB2
2
0
linear
linear
trigonal
planar
AB3
3
0
trigonal
planar
AB4
4
0
tetrahedral
tetrahedral
AB5
5
0
trigonal
bipyramidal
trigonal
bipyramidal
AB6
6
0
octahedral
octahedral
a
b
c
bonding-pair vs. bonding
pair repulsion (a)
‹
lone-pair vs. bonding
pair repulsion (b)
lone-pair vs. lone pair
‹ Repulsion (c)
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB3
3
0
AB2E
2
1
Arrangement of
electron pairs
Molecular
Geometry
trigonal
planar
trigonal
planar
trigonal
planar
bent
18
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB4
4
0
AB3E
3
1
Arrangement of
electron pairs
Molecular
Geometry
tetrahedral
tetrahedral
tetrahedral
trigonal
pyramidal
19
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB4
4
0
Arrangement of
electron pairs
Molecular
Geometry
tetrahedral
tetrahedral
AB3E
3
1
tetrahedral
trigonal
pyramidal
AB2E2
2
2
tetrahedral
bent
20
VSEPR
Class
AB5
AB4E
# of atoms
bonded to
central atom
5
4
# lone
pairs on
central atom
Arrangement of
electron pairs
Molecular
Geometry
0
trigonal
bipyramidal
trigonal
bipyramidal
1
trigonal
bipyramidal
distorted
tetrahedron
21
VSEPR
Class
AB5
# of atoms
bonded to
central atom
5
# lone
pairs on
central atom
0
AB4E
4
1
AB3E2
3
2
Arrangement of
electron pairs
Molecular
Geometry
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
distorted
tetrahedron
T-shaped
22
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB5
5
0
AB4E
4
1
AB3E2
3
2
AB2E3
2
3
Arrangement of
electron pairs
Molecular
Geometry
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
trigonal
bipyramidal
distorted
tetrahedron
trigonal
bipyramidal
linear
T-shaped
23
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
square
pyramidal
Arrangement of
electron pairs
Molecular
Geometry
24
VSEPR
Class
# of atoms
bonded to
central atom
# lone
pairs on
central atom
AB6
6
0
octahedral
octahedral
AB5E
5
1
octahedral
AB4E2
4
2
octahedral
square
pyramidal
square
planar
Arrangement of
electron pairs
Molecular
Geometry
25
26
Dipole Moment and Molecular
Geometry
• The dipole moment is a measure of the
degree of charge separation in a molecule.
We can view the polarity of individual
bonds within a molecule as vector
quantities.
Thus, molecules that are perfectly symmetric
have a zero dipole moment. These
molecules are considered nonpolar.
d+
C
Odd- O
Dipole Moments and Polar Molecules
electron poor
region
electron rich
region
H
F
d+
d-
m=Qxr
Q is the charge, only the magnitude, not to its sign.
r is the distance between charges
1 D (D=debye unit) = 3.36 x 10-30 C m (C is coulomb, m is meter
No electric field
Electric field
Dipole Moment and Molecular
Geometry
:
Molecules that exhibit any asymmetry in the
arrangement of electron pairs would have a nonzero
dipole moment. These molecules are considered polar.
dd-
N
H
H
H
d+
d+
Bond moment (B.M) is the individual
dipole moment (D.M) which is a vector
quantity, has both magnitude and
direction. D.M = sum of vector B.M
The arrow shows
the shift of electron
density from less
electronegative
atom to the more
electronegative
atom
Which of the following molecules have a dipole moment?
H2O, CO2, SO2, and CH4
O
S
dipole moment
polar molecule
dipole moment
polar molecule
H
O
C
O
no dipole moment
nonpolar molecule
H
C
H
H
no dipole moment
nonpolar molecule
Does BF3 have a
dipole moment?
The trigonal planar shape
means that the three bond
moments exactly cancel one
another, makes it a nonpolar
molecule.
Does CH2Cl2 have
a dipole moment?
10.2
Because chlorine is more electronegative than carbon,
which is more electronegative than hydrogen, the bond
moments do not cancel and the molecule possesses a
dipole moment:
Thus, CH2Cl2 is a polar molecule.
Predicting Molecular Geometry
1. Draw Lewis structure for molecule.
2. Count number of lone pairs on the central atom and
number of atoms bonded to the central atom.
3. Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and SF4?
O
S
AB2E
bent
F
O
F
S
F
AB4E
F
distorted
tetrahedron
Predicting Molecular Geometry
• Two electron pairs (linear arrangement).
C
: :
: :
O
O
You have two double bonds, or two electron
groups about the carbon atom.
Thus, according to the VSEPR model, the
bonds are arranged linearly, and the
molecular shape of carbon dioxide is linear.
Bond angle is 180o.
Predicting Molecular Geometry
• Three electron pairs (trigonal planar
arrangement).
:O :
C
: :
: :
:Cl
Cl:
The three groups of electron pairs are
arranged in a trigonal plane. Thus, the
molecular shape of COCl2 is trigonal
planar. Bond angle is 120o.
Predicting Molecular Geometry
:
• Three electron pairs (trigonal planar
arrangement).
O
: :
: :
O
O:
Ozone has three electron groups about the
central oxygen. One group is a lone pair.
These groups have a trigonal planar
arrangement.
Predicting Molecular Geometry
:
• Three electron pairs (trigonal planar
arrangement).
O
: :
: :
O
O:
Since one of the groups is a lone pair, the
molecular geometry is described as bent
or angular.
Predicting Molecular Geometry
:
• Three electron pairs (trigonal planar
arrangement).
O
: :
: :
O
O:
Note that the electron pair arrangement
includes the lone pairs, but the molecular
geometry refers to the spatial arrangement of
just the atoms.
Predicting Molecular Geometry
• Four electron pairs (tetrahedral arrangement).
:
H
:Cl:
C
Cl:
:N
H
:O
:
:
:Cl:
: :
: :
:Cl
H
H
H
Four electron pairs about the central atom
lead to three different molecular
geometries.
Predicting Molecular Geometry
• Four electron pairs (tetrahedral arrangement).
:
H
C
:N
Cl:
:
:Cl:
tetrahedral
H
H
:O
:
: :
: :
:Cl
:Cl:
H
H
Predicting Molecular Geometry
• Four electron pairs (tetrahedral arrangement).
:
:
C
N
H
Cl: H
H
:
:Cl:
H
tetrahedral
trigonal pyramid
:O
:
: :
: :
:Cl
:Cl:
H
Predicting Molecular Geometry
• Four electron pairs (tetrahedral arrangement).
:
C
N
O
: :
:
:
: :
:Cl
:Cl:
Cl: H
H
:
H
:
:Cl:
H
tetrahedral
trigonal pyramid
bent
H
Predicting Molecular Geometry
:
• Five electron pairs (trigonal bipyramidal
arrangement).
:
P
: :
:
:F:
:F
F:
:
F:
:
:F:
:
This structure results in both 90o and 120o
bond angles.
Predicting Molecular Geometry
• Other molecular geometries are possible
when one or more of the electron pairs is a
lone pair.
SF4
ClF3
Let’s try their Lewis structures.
XeF2
Predicting Molecular Geometry
• Other molecular geometries are possible
when one or more of the electron pairs is a
lone pair.
F
F
F
S :
F
see-saw
ClF3
XeF2
Predicting Molecular Geometry
• Other molecular geometries are possible
when one or more of the electron pairs is a
lone pair.
F
F
F
F
S :
:
:
Cl F
F
F
see-saw
T-shape
XeF2
Predicting Molecular Geometry
• Other molecular geometries are possible
when one or more of the electron pairs is a
lone pair.
F
F
F
F
:
Xe :
:
F
see-saw
T-shape
linear
F
F
S
:
:
:
F
Cl F
Predicting Molecular Geometry
• Six electron pairs (octahedral arrangement).
:F:
:
:
S
: :
: :
:F
:
:
:
:F
:F:
F:
F:
:
This octahedral arrangement results in 90o
bond angles.
Predicting Molecular Geometry
• Six electron pairs (octahedral arrangement).
IF5
XeF4
Six electron pairs also lead to other
molecular geometries.
Predicting Molecular Geometry
• Six electron pairs (octahedral arrangement).
F
I
F
F
:
F
F
square pyramid
XeF4
Predicting Molecular Geometry
:
• Six electron pairs (octahedral arrangement).
I
F
F
:
F
F
square pyramid
F
F
Xe
F
F
:
F
square planar
Valence Bond Theory
• Atomic orbitals take part in bond formation by the
overlap of atomic orbitals.
• Eg. H2 and F2 molecules by the overlap of s orbitals of
individual atoms of H and p orbitals of F atoms.
Overlap of orbitals in bond formation
How does Lewis theory explain the bonds in H2 and F2?
Sharing of two electrons between the two atoms.
Overlap Of
Bond Dissociation Energy
Bond Length
H2
436.4 kJ/mole
74 pm
2 1s
F2
150.6 kJ/mole
142 pm
2 2p
Valence bond theory – bonds are formed by sharing
of e- from overlapping atomic orbitals and accounts
for difference in bond enthalpy, bond length etc.
Change in Potential Energy of Two Hydrogen Atoms
as a Function of Their Distance of Separation
Electrons
repels
each
other,
nucleus as
well when
too close.
Most stable, minimum energy, maximum
attraction of nucleus and electrons
Change in electron
density as two hydrogen
atoms approach each
other.
Hybridization – mixing of two or more atomic
orbitals to form a new set of hybrid orbitals
1. Mix at least 2 nonequivalent atomic orbitals (e.g. s
and p). Hybrid orbitals have very different shape
from original atomic orbitals.
2. Number of hybrid orbitals is equal to number of
pure atomic orbitals used in the hybridization
process.
3. Covalent bonds are formed by:
a. Overlap of hybrid orbitals with atomic orbitals
b. Overlap of hybrid orbitals with other hybrid
orbitals
61
Hybrid Orbitals
• Hybrid orbitals are orbitals used to describe
bonding that are obtained by taking
combinations of atomic orbitals of an isolated
atom.
In this case, a set of hybrids are
constructed from one “s” orbital and three
“p” orbitals, so they are called sp3 hybrid
orbitals.
The four sp3 hybrid orbitals take the shape
of a tetrahedron.
sp3 Hybridization
Ground state Carbon atom
↑↓
↑
2s
↑
2p
Excited state Carbon atom
↑
2s
↑
↑
2p
↑
Formation of sp3 Hybrid Orbitals
Methane, CH4
Formation of sp Hybrid Orbitals
Formation of sp2 Hybrid Orbitals
sp2 Hybridization of Carbon
69
Unhybridized 2pz orbital (gray), which is perpendicular
to the plane of the hybrid (green) orbitals.
70
Bonding in Ethylene, C2H4
Sigma bond (s) – electron density between the 2 atoms
Pi bond (p) – electron density above and below plane of nuclei
of the bonding atoms
71
Sigma (s) and Pi Bonds (p)
Single bond
1 sigma bond
Double bond
1 sigma bond and 1 pi bond
Triple bond
1 sigma bond and 2 pi bonds
72
Another View of p Bonding in Ethylene, C2H4
73
sp Hybridization of Carbon
74
Experiments show O2 is paramagnetic
O
O
No unpaired eShould be diamagnetic
Molecular orbital theory – bonds are formed from
interaction of atomic orbitals to form molecular
orbitals.
75
Energy levels of bonding and antibonding molecular
orbitals in hydrogen (H2).
A bonding molecular orbital has lower energy and greater
stability than the atomic orbitals from which it was formed.
An antibonding molecular orbital has higher energy and
lower stability than the atomic orbitals from which it was
formed.
76
Two Possible Interactions Between Two Equivalent p Orbitals
77
Molecular Orbital (MO) Configurations
1. The number of molecular orbitals (MOs) formed is always
equal to the number of atomic orbitals combined.
2. The more stable the bonding MO, the less stable the
corresponding antibonding MO.
3. The filling of MOs proceeds from low to high energies.
4. Each MO can accommodate up to two electrons.
5. Use Hund’s rule when adding electrons to MOs of the
same energy.
6. The number of electrons in the MOs is equal to the sum of
all the electrons on the bonding atoms.
78
1
bond order =
2
bond
order
½
(
1
Number of
electrons in
bonding
MOs
-
Number of
electrons in
antibonding
MOs
½
0
)
79
General molecular orbital energy level diagram for the
second-period homonuclear diatomic molecules Li2, Be2, B2,
C2, and N2.
80
81
Delocalized molecular orbitals are not confined between
two adjacent bonding atoms, but actually extend over three
or more atoms.
Example: Benzene, C6H6
Delocalized p orbitals
82
Homework-Ch-10
10.2,10.8,10.9,10.20,10.24,10.30,
10.32,10.42,10.46,10.49,10.53,10.56,10.58