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Chemical Bonding II:Molecular Geometry and
Hybridization of Atomic OrbitalsChapter 10
Linear180o
Trigonalplanar120o
Tetrahedral 109.5o
TrigonalBipyramidal120 and 90o
Octahedral 90o
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linear Linear(AB2)
Trigonal planar
Trigonal planar(AB3)
Bent(AB2E)
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TetrahedralBent
(AB2E2)
Tetrahedral(AB4)
Pyramidal(AB3E)
Trigonal Bipyramidal
TrigonalBipyramidal
(AB5)
Unsymmetrical Tetrahedron
(AB4E)
T-shaped(AB3E2)
Linear(AB2E3)
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Octahedral Square planar(AB4E2)
Octahedral(AB6)
Square pyramidal
(AB5E)
bonding-pair vs. bondingpair repulsion
lone-pair vs. lone pairrepulsion
lone-pair vs. bondingpair repulsion
> >
Lone Pairs Distort Bond Angles
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1.Determine the Lewis structure
2.Determine the number of electron pairs (or clouds) around the CENTRAL ATOM –multiple bonds count as ONE CLOUD (see next slide).
3.Find out the appropriate VSEPR geometry for the specified number of electron pairs, both bonding and lone pairs.
4.Use the positions of atoms to establish the resulting molecular geometry.
Multiple Bonds and Molecular Geometry
Multiple bonds count as one -
e.g. 4 bonding pairs around C, but trigonal planar instead of tetrahedral.
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LINEAR - AB2 e.g. BeCl2
TRIGONAL PLANAR - BF3 (AB3)- SO2 (AB2E)
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TETRAHEDRAL - CH4 (AB4)- NH3 (AB3E)- H2O (AB2E2)
TRIGONAL BIPYRAMIDAL - PCl5 (AB5)- SF4 (AB4E)- ClF3 (AB3E2)- I3- (AB2E3)
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OCTAHEDRAL - SF6 (AB6)- BrF5 (AB5E)- XeF4 (AB4E2)
Example 10.1 Use the VSEPR model to predict the geometry of the following molecules and ions: (a) AsH3, (b) OF2, (c) AlCl4-, (d) I3-, (e) C2H4
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Molecular Geometries of Molecules with more than 1 Central Atom
cysteine
Dipole Moments and Polar Molecules
H F
electron richregionelectron poor
region
δ+ δ−
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10.2
CCl4 vs CHCl3
C ClCl
Cl
Cl
C ClCl
H
Cl
Which molecule is polar? You must look at the geometry to decide.
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Cl2CO
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NH3
H2O
These types of molecules, where C = central atom and T = terminal atoms of the same type, are never polar.
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The Orbital Overlap Model of Bonding
End to end overlap = sigma (σ) bond
H-H H-F
Valence Bond Theory (hybrid orbitals)
109.5 o
Lewis Structure
Predicted Bonding and VSEPR Geometry for CH4
Electron pairs around C
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Problem: the available s and p-orbitals are at 90o angles,not at the predicted 109.5o!
New orbitals are constructed from pre-existing s, p, and d-orbitals = hybrid orbitals
1. Hybridize the CENTRAL ATOM ONLY (others asneeded)
2. Only use valence shell electrons
3. The number of hybrid orbitals formed = number ofatomic orbitals used
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For CH4, we need 4 hybrid orbitals, so 4 atomic orbitalsare required as follows: (s + p + p + p) = sp3
Needed to form 4 sigma bonds
Fig. 10.7
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Fig. 10.8
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(will be upgraded as we proceed)
1. Hybrid orbitals get 1 electron for a σ-bond, 2 electronsfor a lone pair.
sp3 hybridization for H2O
Needed to form 2 sigma bonds and 2 lone pairs
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BF3 - trigonal planar according to VSEPR Theory (incomplete octet exception)
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For BF3, we need 3 hybrid orbitals, so 3 atomic orbitalsare required as follows: (s + p + p) = sp2
Needed to form 3 sigma bonds
BeCl2 - linear according to VSEPR Theory
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For BeCl2, we need 2 hybrid orbitals, so 2 atomic orbitalsare required as follows: (s + p) = sp
Needed to form 2 sigma bonds
Ex 10.4 Describe the hybridization state of phosphorus in PBr5
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For PBr5, we need 5 hybrid orbitals, so 5 atomic orbitalsare required as follows: (s + p + p + p + d) = sp3d
Needed to form 5 sigma bonds
e.g. SF6
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For SF6, we need 6 hybrid orbitals, so 6 atomic orbitalsare required as follows: (s + p + p + p + d + d) = sp3d2
Isolated S atom Needed to form 6 sigma bonds
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Sigma (σ) bonds = end-to-end overlap
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Pi (π) bond = side-by-side overlap
C - C 1 σ bond
C = C 1 σ bond1 π bond
C C 1 σ bond2 π bonds
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(upgraded – more will be added)
1. Hybrid orbitals get 1 electron for a σ-bond, 2 electronsfor a lone pair.
2. Remaining electrons go into unhybridized orbitals= π bonds
DOUBLE BONDS: Ethylene, CH2CH2
Lewis Structure:
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H HC = C
H H
Apply VSEPR Theory and Determine Hybridization
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sp2 hybridization on each C atom -
sp2 hybrids and unhybridized p-orbital
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σ bond = end-to-end overlap of the sp2
hybridized orbitals
••
••• ••• • •
1 electron from the sp2 hybrid on C, the other from the hydrogen 1s orbital
• •
π bond = side-by-side overlap of theunhybridized p-orbitals
Electron from the unhybridizedp-orbital on the C atom
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Sigma (σ) Bonding in Ethylene
Pi (π) Bonding in Ethylene
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DOUBLE BONDS: Formaldehyde, CH2O
Lewis Structure:
Apply VSEPR Theory and Determine Hybridization
HC = O
H
• •
• •
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sp2
120 o
sp2 hybridization on C -
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sp2 hybridization on O -
Sigma (σ) Bonding in Formaldehyde
••
••• •
• •
••
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Pi (π) Bonding in FormaldehydeElectron from the unhybridizedp-orbitals
• •
TRIPLE BONDS: Acetylene, C2H2
Lewis Structure:
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Apply VSEPR Theory and Determine Hybridization
H - C C - H
sp hybridization on each C atom -
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sp hybrids and unhybridized p-orbitals
Sigma (σ) Bonding in Acetylene
Unhybridized p-orbitals
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Pi (π) Bonding in Acetylene
Explain the Bonding Using Valence Bond Theory
CO2
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Sigma Bonding in CO2
Pi Bonding in CO2
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VSEPR Theory - electron pair repulsions influence molecular shape
Valence Bond Theory - atoms form bonds by overlapping atomic and/or hybrid orbitals
Applied to O2 - 2(6) = 12 valence electrons or 6 pairs
O = O• •
• • • •• •
O = O• •
• • • •• •
This prediction is WRONG! Since all of the electrons are paired up, the molecule should be diamagnetic, but experiments prove that it is PARAMAGNETIC!
An additional refinement in bonding theory is necessary =
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Molecular Orbitals - Preliminary Ideas
Don’t forget that electrons behave like WAVES, and there are WAVE FUNCTIONS (ψ) that describe the electron position in space = ATOMIC ORBITALS (ψ2)
e'
Waves (electrons) can interfere with each other, either CONSTRUCTIVELY or DESTRUCTIVELY
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Molecular Orbitals - destructive and constructive interference of atomic orbitals
Sigma bond formation involving p-orbitals
σ*2p
σ2p
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Pi bond formation involving p-orbitals
π*2p
π*2p
π2p
π2p
Principles of Molecular Orbital Theory
1. The total number of molecular orbitals = total numberof atomic orbitals contributed by the bonding atoms
2. Bonding MO’s are lower in energy (more stable) thanantibonding MO’s
3. Electrons occupy molecular orbitals following thePauli Exclusion Principle (spins pair up) and Hund’sRule (remain unpaired as long as an empty orbital isavailable of the same energy)
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Energy Levels of Molecular Orbitals for Homonuclear Diatomics - H2, O2, etc
Molecular orbitals
Atomic orbitals
Atomic orbitals 2p2p
2s2s
1s1s
σ1s
σ*1s
σ2s
σ*2s
π2p
σ2p
π*2p
σ*2p
Molecular Orbital Electron Configurations
e.g. O2
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Bond Order
Order = ½ [# electrons bonding MO’s -# electrons antibonding MO’s]
1. The greater the bond order, the more stable the molecule
2. A high bond order means higher bond energies andshorter bond lengths.
3. Fractional bond orders are possible
σ1s
σ*1s
1s1s
H2+
σ1s
σ*1s
1s1s
H2
Bond order = Bond order =
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σ1s
σ*1s
1s1s
σ1s
σ*1s
1s1s
He2He2+
Bond order = Bond order =
Homonuclear Diatomic Molecules of Second Row Elements(the inner MO’s formed from overlap of the 1s orbitals aren’t shown)
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Let’s take a look at the molecule ozone - O3
Lewis structure: 3(6) = 18 or 9 pairs
sp2 hybridization of the central oxygen -
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sp2 hybridization of the terminal oxygens -
Sigma Bonding in O3
Explain using Valence Bond Theory
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Pi Bonding in O3
Combine 3 p-orbitals = 3 molecular orbitals
Pi Bonding in O3
Antibonding π orbital Nonbonding π orbital
••
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Bonding π orbital
Electrons in the bonding MO are free to move along the length of the molecule = delocalization
• •
Another example - NO3-
O - N - O| | O
• •• •
• •• •
••••
• • • •O = N - O|
O
• •• •
• •• •
• • ••••
• • O - N = O|
O ••••
• •• •
• •
• •• •
••
- - -
Hybridize all of the atoms to sp2 and combine the unused p-orbitals into molecular orbitals.
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Benzene - C6H6
sp2 hybridize the C atoms and combine the unused p-orbitals into molecular orbitals.
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