AP Chem: Notes chapters 7, 8, 9
Chapter 9: COVALENT BONDING: ORBITALS
What molecule is this?
Hybridization: The mixing of atomic orbitals to form special molecular orbitals for bonding.
The atoms are responding as needed to give the minimum energy for the molecule.
Molecular Geometry and Hybridization:

Parent geometry determines the hybridization.

Molecular structure is the actual geometry.
sp3 HYBRIDIZATION
Energy-level diagram showing the formation of four sp3 hybrid orbitals.
One 2s and three 2p orbitals hybridize to form a new set of sp3 hybrid orbitals.
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z
z
z
x
x
y
y
s
z
x
Hybridization y
px
z
x
y
py
x
y
z
x
y
z
x
y
pz
z
x
y
sp3
gives a tetrahedral
arrangement
sp3
sp3
sp3
sp3
•
•
•
hybrid orbital:
4 effective electron pairs.
tetrahedral geometry.
109.5° bond angle.
The tetrahedral set of four sp3 orbitals of the carbon atom share one electron each with the
four hydrogen atoms to make a methane molecule.
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H1s
sp3
sp3
H1s
C
sp3
sp3
H1s
H1s
The nitrogen atom in ammonia is sp3 hybridized.
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lone pair
sp
3
sp
3
N
H1s
H1s
sp
3
sp
3
H1s
A sigma (σ) bond centers along the internuclear axis.
 bonds consist of an electron pair shared in the area centered between the atoms.
 bonds allow rotation.
 bonds in a molecule are described as being localized.
A pi (π) bond occupies the space above and below the internuclear axis.
 bonds occupy the space above and below a line joining the atoms.
 bonds do not allow rotation.
 bonds are considered to be delocalized over the entire molecule.
The orbitals used to form the bonds in ethylene:
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sp 2
H 1s
sp 2
sp 2
sp 2
H 1s
C
C
2p
sp 2
sp 2
(a)
H
H
(b)
H
C
H
C
sp2 HYBRIDIZATION
An orbital energy-level diagram for sp2 hybridization.
In sp2 hybridization one p orbital remains unchanged and lies perpendicular to the plane of the
hybrid.
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p orbital
sp2 orbital
sp2 orbital
sp2 orbital
The shared electron pair of in ethylene occupies the region directly between the atoms to
form a sigma () bond.
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H1s
H1 s
sp 2
sp 2
C
C
sp 2
H1s
sp 2
sp 2
sp 2
H1 s
sp2
•
•
•
hybrid orbital
three effective electron pairs.
trigonal planar geometry.
120° bond angle.
A carbon-carbon double bond consists of a  bond and a  bond. The  bond is formed from
unhybridized p orbitals in the space above and below the  bond.
p orbital
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p orbital
C
pi bond
C
sigma
bond
Two sp orbitals are formed when one s and one p orbital are hybridized. They are oriented
at 180° to each other.
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z
z
z
z
z
Hybridization
x
y
s
x
y
px
x
y
x
y
gives a linear
arrangement
180°
x
y
sp HYBRIDIZATION
The hybrid orbitals in the CO2 molecule.
sigma bond
(1 pair of electrons)
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O
pi bond
(1 pair of
electrons)
C
O
pi bond
(1 pair of
electrons)
(a)
O
C
(b)
sp hybrid orbital
• 2 effective electron pairs.
• linear geometry.
• 180° bond angle.
O
The nitrogen molecule forms a triple bond -- one  and two  bonds.
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p
sp
N
sp
p
(a)
lone pair sigma bond lone pair
N
sp
N
sp sp
sp
(b)
N
N
(c)
dsp3 hybrid orbitals
five effective electron pairs.
trigonal bipyramidal geometry.
90° and 120° bond angles.
hybrid orbitals are not all equivalent as in the other
types of hybridization.
• Phosphorus pentachloride
•
•
•
•
d2sp3 hybrid orbitals
•
•
•
•
six effective electron pairs.
octahedral geometry.
90° bond angles.
Sulfur hexafluoride
The relationship of the number of effective pairs, their spatial arrangement, and the hybrid
orbitals.
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Number of
Effective Pairs
Arrangement
of Pairs
Hybridization
Required
180°
2
Linear
sp
3
Trigonal
planar
sp2
120°
109.5°
4
Tetrahedral
5
Trigonal
bipyramidal
sp3
90°
dsp3
120°
90°
6
Octahedral
d2sp3
90°
The Localized Electron Model



Draw the Lewis structure(s)
Determine the arrangement of electron pairs (VSEPR model).
Specify the necessary hybrid orbitals.
Deficiencies of the LEM Model




Does not adequately explain resonance.
Does not work for odd-electron molecules and ions.
Assumes that all electrons are localized about an atom.
Gives no direct information about bond energies.
Molecular Orbitals (MO)
Analagous to atomic orbitals for atoms, MOs are the quantum mechanical solutions to the
organization of valence electrons in molecules. Electrons are considered to be delocalized over the
entire molecule.
Types of MOs
bonding: lower in energy than the atomic orbitals from which it is composed.
antibonding: higher in energy than the atomic orbitals from which it is composed.
The molecular orbital energy diagram for the H2 molecule and the MO1 and MO2 orbitals
formed.MO1 = σ1s and MO2 = σ1s*.
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Energy diagram
Electron probability distribution
HA
H2
HB
MO2
E
1s A
+
+
+
+
1s B
MO1
(b)
(a)
Bond Order
Difference between the number of bonding electrons and number of antibonding electrons
dividedby two.
BO =
# bonding electrons  # antibonding electrons
2
Larger bond order means greater bond strength!
The molecular orbital energy-level diagrams, bond orders, bond energies, and bond lengths for
diatomic molecules.
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B2
C2
N2
O2
2p*
 2p*
2p*
 2p*
2p
 2p
2p
 2p
2s*
 2s*
2s
 2s
F2
E
Magnetism
Para–
magnetic
Dia–
magnetic
Dia–
magnetic
Para–
magnetic
Dia–
magnetic
Bond order
1
2
3
2
1
Observed
bond
dissociation
energy
(kJ/mol)
290
620
942
495
154
Observed
bond
length
(pm)
159
131
110
121
143
In order to participate in MOs, atomic orbitals must overlap in space.
(Therefore, only valence orbitals of atoms contribute significantly to MOs.)
The relative size of the lithium 1s and 2s orbitals. The 1s orbital can be considered to be
localized and do not participate in bonding.
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Li
Li
1s
1s
2s
2s
The boron molecule will form one  and two  bonds.
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B
B
(a)
(b)
(c)
(d)
The two p orbitals that overlap head on make two  molecular orbitals -- one bonding and one
antibonding. The two p orbitals that lie parallel overlap to produce two  molecular orbitals,
one bonding and one antibonding.
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*2p
Antibonding
2px
2px
(a)
2p
Bonding
*2p
Antibonding
2py
2p
2py
(b)
Bonding
Paramagnetism: (see previous diagram of The molecular orbital energy-level diagrams, bond orders,
bond energies, and bond lengths for diatomic molecules.)
- unpaired electrons
- attracted to induced magnetic field
- much stronger than diamagnetism
- B2 & O2
Dimagnetism:
- paired electrons
- repelled from induced magnetic field
- much weaker than paramagnetism
- C2, N2 & F2
Apparatus used to measure the paramagnetism of a sample. A paramagnetic sample will
appearheavier when the electromagnet is turned on.
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Balance
Glass tubing
Sample tube
Electromagnet
A partial molecular orbital energy-level diagram for the HF molecule. Bond order is 1 -- a
single bond.
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H
atom
HF
molecule
F
atom
*
1s
E
2p

The electron probability distribution in the bonding molecular orbital of the HF molecule.
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H nucleus
F nucleus
NO2 Molecule
Draw the Lewis Structure, determine the parent geometry, the actual geometry, and
the approximate bond angle.
NO+ ION
Draw the Lewis Structure, draw the molecular orbital energy-level diagram, determine the
bond order, and the type of magnetism for the NO+ ion.
The  bonding system in the benzene molecule.
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sp 2
H
C
C
H
H
sp 2
C
H
C
H1s
C
C
H
H
The  molecular orbital system in benzene. The electrons in the  orbitals are delocalized
over the ring of carbon atoms.
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H
H
H
H
H
H
H
H
H
(a)
H
H
H
(b)
Outcomes of MO Model
1.
As bond order increases, bond energy increases and bond length decreases.
2. Bond order is not absolutely associated with a particular bond energy.
3. N2 has a triple bond, and a correspondingly high bond energy.
4. O2 is paramagnetic. This is predicted by the MO model, not by the LE model, which predicts
diamagnetism.
Combining LE and MO Models
 σ bonds can be described as being localized.
 π bonding must be treated as being delocalized.