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 & Hybridization
Parent geometry determines the hybridization.
Molecular structure is the actual geometry.
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z
y
z
x
y
x
x
y
z
y
x
sp3
sp3
sp3
sp3
Hybridization
gives a tetrahedralarrangement
s
p y
p x
p z
x
y
z
z
y
x
y
z
x
y
z
z
x
One 2s and three 2p orbitals hybridize to form a new set of sp3 hybrid orbitals.
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sp3
sp3
H1s
H1s H1s
H1s
C
sp3
sp3
The tetrahedral set of four sp3 orbitals of thecarbon atom share one electron each with the four hydrogen atoms to make a methane molecule.
A sigma () bond centers along the internuclear axis.
A pi () bond occupies the space above and below the internuclear axis.
CCH H
HH
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E
2p
2s p
Orbitals in an isolatedcarbon atom
sp2
Carbon orbitals in ethylene
Hybridization 2p
An orbital energy-level diagram for sp2 hybridization.
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sp2 orbital
p orbital
sp2 orbital
sp2 orbital
In sp2 hybridization one p orbital remains unchangedand lies perpendicular to the plane of the hybrid.
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C C
sp 2
sp 2
sp2
sp2
H1s
H1s
H1s
H1s sp2sp2
The shared electron pair of in ethylene occupiesthe region directly between the atoms to form a sigma () bond.
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sigmabond
pi bondC C
p orbital p orbital
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.
Pi and Sigma Bonds
bonds consist of an electron pair shared in the area centered between the atoms.
bonds occupy the space above and below a line joining the atoms.
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(b)
H
C C
H
H
H
sp 2
sp 2
sp 2
sp 2
H1sH1s C C
2p
sp 2 sp 2
(a)
The orbitals used to form the bonds in ethylene.
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z
s px yy
x
zz
y
gives a lineararrangement
x
y
z
Hybridization180°
xx x
y
z
Two sp orbitals are formed when one s and one p orbitalare hybridized. They are oriented at 180o to each other.
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O C O
sigma bond(1 pair of electrons) pi bond
(1 pair ofelectrons)
pi bond(1 pair ofelectrons)
(a)
(b)
O C O
The hybrid orbitals in the CO2 molecule.
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(c)
NN
p
spsp
p
(a)
N
lone pair sigma bond lone pair
NN
(b)
sp sp sp sp
The nitrogen molecule forms a triple bond -- one and two bonds.
dsp3 hybrid orbitals
• five effective electron pairs.• trigonal bipyramidal geometry.• 90 o and 120 o 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 o bond angles.• Sulfur hexafluoride.
09_179 Number ofEffective Pairs
Arrangementof Pairs
HybridizationRequired
2 Linear sp
180°
3 Trigonalplanar
sp2
120°
4 Tetrahedral
109.5°
5 Trigonalbipyramidal
dsp3
90°
120°
90°
90°
6 Octahedral d2sp3
sp3
The relationship of the number of effective pairs,their spatial arrangement, and the hybrid orbitals.
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.
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EMO2
1sA
H2HA HB
1sB
MO1
Energy diagram
(a)
Electron probability distribution
+ +
+ +(b)
The molecular orbital energy diagram for the H2 molecule and the MO1 and MO2 orbitals formed.MO1 = 1s and MO2 = 1s*.
Bond Order (BO)
Difference between the number of bonding electrons and number of antibonding electrons divided by two.
# bonding electrons # antibonding electronsBO =
2
Larger bond order means greater bond strength!
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E
2p*
2p*
2p
2p
2s*
2s
B 2 C2 N 2 O2 F2
MagnetismPara–
magneticDia–
magneticDia–
magneticPara–
magneticDia–
magnetic
Bond order 1 2 3
2s
2s*
2p
2p
2p*
2p*
2 1
Observedbonddissociationenergy(kJ/mol) 290 620 942 495 154
Observed bondlength(pm) 159 131 110 121 143
The molecular orbital energy-level diagrams, bond orders,bond energies, and bond lengths for diatomic molecules.
In order to participate in MOs, atomic orbitals must overlap in space. (Therefore, only valence orbitals of atoms contribute significantly to MOs.)
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Li
1s
2s
Li
1s
2s
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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2py 2py
2px 2px
Antibonding
Bonding
2p
*2p
Antibonding
Bonding
*2p
2p
(b)
(a)
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.
Paramagnetism
- unpaired electrons
- attracted to induced magnetic field
- much stronger than diamagnetism
- B2 & O2
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E
2p*
2p*
2p
2p
2s*
2s
B 2 C2 N 2 O2 F2
MagnetismPara–
magneticDia–
magneticDia–
magneticPara–
magneticDia–
magnetic
Bond order 1 2 3
2s
2s*
2p
2p
2p*
2p*
2 1
Observedbonddissociationenergy(kJ/mol) 290 620 942 495 154
Observed bondlength(pm) 159 131 110 121 143
The molecular orbital energy-level diagrams, bond orders,bond energies, and bond lengths for diatomic molecules.
Diamagnetism
- paired electrons
- repelled from induced magnetic field
- much weaker than paramagnetism
- C2 , N2 , & F2 .
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Glass tubing
Sample tube
Electromagnet
Balance
Apparatus used to measure the paramagnetismof a sample. A paramagnetic sample will appearheavier when the electromagnet is turned on.
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E
1s
HFmolecule
Hatom
Fatom
2p
*
A partial molecular orbital energy-level diagramfor the HF molecule. Bond order is 1 -- a single bond.
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H nucleus F nucleus
The electron probability distribution in the bondingmolecular orbital of the HF molecule.
Pi and Sigma Bonds
bonds in a molecule are described as being localized.
bonds are considered to be delocalized over the entire molecule.
NO2 Molecule
Draw the Lewis Structure, determine the parent geometry, the actual geometry, and the approximate bond angle.
Draw the Lewis Structure, draw themolecular orbital energy-level diagram, determine the bond order, and the type ofmagnetism for the NO+ ion.
NO+ ION
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H H
H
H
H H
H H
H H
(a) (b)
H
H
The molecular orbital system in benzene. Theelectrons in the orbitals are delocalized over thering of carbon atoms.
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.