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Chapter Ten
Chemical Bonding ll
Molecular Geometry and
Hybridization of Atomic Orbitals
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Molecular Geometry
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The Valence-Shell Electron-Pair Repulsion (VSEPR)
Method based on the idea that pairs of valence
electrons in bonded atoms repel one another.
Assumes electron pairs try to get as far apart as possible
Each electron pair or bond takes up ~ same amount of space
# of bonds or pairs determines molecular geometry
Molecular Geometry:
The shape of a molecule that
describes the location of nuclei
& the connections between them.
Molecules with No Lone Pairs
Bond angles due to # of repulsions
Each bond takes up space of 1 electron pair
AB2
AB3
Linear
Trigonal planar
AB5
Trigonal bipyramid
AB4
Tetrahedral
AB6
Octahedral
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Molecules with Lone Pairs: Know table 10.2
Lone pair electrons not seen but take up space
Act as “invisible bond”
Count electrons as E’s
Single, double or triple bonds count as 1 bond
To determine molecular geometry
Add up all the B’s and E’s on the molecule


H O  H
B A
B
AB2E2

The sum equals number of spaces needed
2B + 2E = 4
# spaces = 4
Match to table of geometries without lone pairs

Electron pair geometry: Tetrahedral
Molecular Geometry: Bent
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Molecules with More than 1 Central Atom
VSEPR must be done separately for each atom
May result in a different molecular geometry around each one
Methanol CH3OH
C: 4 spaces: tetrahedral
O: 4 spaces: bent
H
H
C
O
H
H
Oxoacids: Hydrogen goes on oxygens
HNO3, H2SO4, etc. will also use this method
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Dipole Moments
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Dipole Moments and Polar Molecules
electron poor
region
electron rich
region
H
F


= Q x r
Q is the charge
r is the distance between charges
1 D = 3.36 x 10-30 C m
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Predicting Polarity: CO2
Predict molecular shape
VSEPR
Linear
AB2
O=C=O
Predict bond dipoles
C less electronegative than O
C-O
Bond dipoles cancel or combine?
Cancel
Nonpolar
O=C=O
=0
Predicting Polarity: NH3
Predict molecular shape.
VSEPR
Tetrahedral
AB3E
Predict bond dipoles.
H less electronegative than N
N-H
lone pair more electronegative than N
Bond dipoles cancel or combine?
Combine: Polar molecule
 >0
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Polarity of different Molecules
Isomers:
Same molecular formula
Different structure
Cis
Large groups on same
side of double bond plane
Trans
Large groups across
plane of double bond
Dichloroethylene: C2H2Cl2
2 possible isomers
Cis-dichloroethylene
Trans-dichloroethylene
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Valence Bond Theory
and
Hybridization
Valance Bond Theory: Formation of H2(g)
Covalent bond formation
Electrons in 1s atomic orbitals pair
Opposing spins occupy the overlap
region between 2 atoms
Shield nuclei from each other
Delocalization
Area of high electron density (red)
Lowers energy, provides stability
Bonding electrons are found in the
overlap region (covalent bond)
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Carbon Bonding
Electronic configuration
C should have 2 bonds
2 half-filled orbitals on C
[He] 2s22px12py12pz0
Experimentally
C has 4 identical bonds: CH4
Implies 4 half-filled orbitals
[He] 2s12px12py12pz1
Excite 2s1 electron to 2pz orbital
Problems with Theory
4 bonds, but orbitals of differing energies, bond lengths
3 bonds: H
1s
C
2p
Higher energy
1 bond: H
1s
C
2s
Lower energy
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Hybrid Orbitals
Model that predicts shape based on atomic orbitals
Allows use of electrons in s, p and d orbitals in bond
Creates several identical bonds
“Averages” orbital energies to equalize bonds
Used for central atoms in covalent bonds
# hybrid orbitals = # combining atomic orbitals
Use VSEPR and Lewis theory to predict geometry
Determine Lewis structure and VSEPR notation
Orientation of determines electron geometry
Determine hybridization based on VSEPR model
Electron pairs may occupy 1 or more of the hybrid orbitals
sp3 Hybridization
4 equivalent orbitals
1part s to 3 parts p: sp3
CH4 : 4 valence electrons
NH3 : 5 valence electrons
Orbitals point toward corners of tetrahedron
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sp2 Hybridization
3 sp2 hybrid orbitals in a plane
1 2s orbital & 2 2p orbitals
Forms 3 sp2 hybrids with 1 empty 2p orbital
Trigonal planar geometry: 120o angles.
Often involves double bonds
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sp Hybridization
Two sp orbitals in a plane
1 2s orbital & 1 2p orbitals
Forms 2 sp hybrids and 1 empty 2p orbital
Linear geometry: 180o angles
Triple bonds may be present
BE: 2 2s valence electrons
2 sp hybrid orbitals
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Hybrid Orbitals Involving d Subshells
Allows for expanded valence shell compounds
3s electron promoted to 3d subshell
Five sp3d hybrid orbitals.
Trigonal bipyramidal molecular geometry
3s & 3p electrons promoted to 3d subshell
Six sp3d2 hybrid orbitals
Octahedral molecular geometry.
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Hybrid Orbitals: d and f Subshells
Allows for expanded valence shell compounds
A 3s and a 3p electron are promoted to 3d subshell
Makes 6 sp3d2 hybrid orbitals
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Bonding Scheme for Iodine Pentafluoride
(IF5)
VSEPR
AX5E
Electron Geometry
Octahedron
Molecular Geometry
Tetragonal Pyramid
Bonding
5 sp3d2 I - F bonds
1 electron pair in sp3d2 orbital
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Hybrid Orbitals and Geometric Orientations
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Hybridization
of
Double and Triple Bonds
Carbon Bonding: sp2 Hybridization of CH2O
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10.5
Sigma and Pi Bonding in Ethylene
Ethylene
Sigma Bonding ()
End to end
s,p (or d) orbitals
Single bonds
Pi Bonding ()
Parallel side to side
C: 3 sp2 orbitals
H: 1s orbital
C: 1 p orbital
Double bond
1e- from each C
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Bonding in Acetylene, C2H2
sp
Hybridization
Sigma Bonding
End to End
C: 2 sp orbitals
H: 1s orbital
HC  CH
Pi Bonding
Side to Side
2 p orbitals per C
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Molecular Orbital Theory
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Molecular Orbitals and Bonding In H2 (1s orbitals)
Molecular orbitals (MOs)
Orbitals that result from the interaction of atomic orbitals
Interaction can stabilize or destabilize molecule
Higher energy than atomic orbitals
No electron density in center.
Bonding Orbital
Lower energy than atomic orbitals: High charge density in center
Antibonding Orbital
Higher in energy, designated with a *
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Molecular Orbitals From 2p Atomic Orbitals
6 atomic orbitals
6 molecular orbitals
Bond Order: Can the Molecule Exist?
BO = ½ (# bonding electrons - # antibonding electrons)
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