Transcript CH 10: Molecular Geometry

CH 10: Molecular Geometry
Renee Y. Becker
Valencia Community College
CHM 1045
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Molecular Shapes: VSEPR
• The approximate shape of molecules is given by
Valence-Shell Electron-Pair Repulsion (VSEPR).
• Step 01: Count the total electron groups.
• Step 02: Arrange electron groups to
maximize separation.
• Groups are collections of bond pairs between
two atoms or a lone pair.
• Groups do not compete equally for space:
Lone Pair > Triple Bond > Double Bond > Single
Bond
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Molecular Shapes: VSEPR
• Two Electron Groups: Electron groups
point in opposite directions.
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Molecular Shapes: VSEPR
• Three Electron Groups: Electron
groups lie in the same plane and point
to the corners of an equilateral triangle.
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Molecular Shapes: VSEPR
• Four Electron
Groups:
• Electron groups
point to the
corners of a
regular
tetrahedron.
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Molecular Shapes: VSEPR
Five Electron Groups: Electron groups point to
the corners of a trigonal bipyramid.
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Molecular Shapes: VSEPR
• Six Electron
Groups:
Electron groups
point to the
corners of a
regular
octahedron.
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Molecular Shapes: VSEPR
Electron Groups
Lone Pairs
Bonds
Geometry
Examples
2
0
2
Linear
BeCl2
3
0
3
Trigonal planar
BF3
3
1
2
Bent
SO2
4
0
4
Tetrahedral
CH4
4
1
3
Trigonal pyramidal
NH3
4
2
2
Bent
H2O
5
0
5
Trigonal bipyramidal
PCl5
5
1
4
See-saw
SF4
5
2
3
T-Shaped
ClF3
5
3
2
linear
I3-
6
0
6
Octahedral
SF6
6
1
5
Square pyramidal
SbCl52-
6
2
4
Square planar
XeF4
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Example 1: VSEPR
• Draw the Lewis electron-dot structure and
predict the shapes of the following molecules
or ions:
O3
H3O+
XeF2
PF6–
XeOF4
AlH4–
BF4–
SiCl4
ICl4–
AlCl3
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Valence Bond Theory
1. Covalent bonds are formed by overlapping of
atomic orbitals, each of which contains one
electron of opposite spin.
2. Each of the bonded atoms maintains its own
atomic orbitals, but the electron pair in the
overlapping orbitals is shared by both atoms.
3. The greater the amount of orbital overlap, the
stronger the bond.
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Valence Bond Theory
• Linus Pauling: Wave functions from s
orbitals & p orbitals could be combined
to form hybrid atomic orbitals.
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• sp hybrid:
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• sp2 hybrid:
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• sp2 hybrid (π bond):
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• sp3 hybrid:
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sp3d hybrid:
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• sp3d2 hybrid:
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Hybridization Easy Way
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Molecular Orbital Theory
• The molecular orbital (MO) model provides
a better explanation of chemical and physical
properties than the valence bond (VB) model.
– Atomic Orbital: Probability of finding the
electron within a given region of space in
an atom.
– Molecular Orbital: Probability of finding the
electron within a given region of space in a
molecule.
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Molecular Orbital Theory
• Additive combination of orbitals (s) is
lower in energy than two isolated 1s
orbitals and is called a bonding molecular
orbital.
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Molecular Orbital Theory
• Subtractive combination of orbitals
(s*) is higher in energy than two isolated
1s orbitals and is called an antibonding
molecular orbital.
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Molecular Orbital Theory
• Molecular Orbital Diagram for H2:
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Molecular Orbital Theory
• Molecular Orbital Diagrams for H2–
and He2:
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Molecular Orbital Theory
• Additive and subtractive combination of
p orbitals leads to the formation of both
sigma and pi orbitals.
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Molecular Orbital Theory
• Second-Row MO Energy Level
Diagrams:
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Molecular Orbital Theory
• MO Diagrams Can Predict Magnetic
Properties:
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Molecular Orbital Theory
• Bond Order is the number of electron pairs
shared between atoms.
• Bond Order is obtained by subtracting the
number of antibonding electrons from the
number of bonding electrons and dividing by
2.
BO = Bonding electrons – antibonding electrons
2
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Example 2: Molecular Orbital Theory
• The B2 and C2 molecules have MO
diagrams similar to N2. What MOs are
occupied in B2 and C2, and what is the
bond order in each? Would any of these
be paramagnetic?
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s*
B2
2p
*2p
B has 5 electrons
So B2 has 10 elec
s2p
2p
Core electrons don’t
count toward BO
s*2s
s2s
s*1s
s1s
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s*2p
*2p
s2p
C2
Carbon has 6
electrons so C2 has
12 electrons
2p
s*2s
s2s
s*1s
s1s
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Dipole moment
• Based on electronegativity differences between atoms in
a molecule
• The most electronegative atom is partially negative
• The less electronegative atom is partially positive
• The dipole moment is the average of all dipoles in the
molecule
• Exception (C-H bonds do not have a dipole)
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• If a molecule has a dipole moment it is
polar
• A compound could contain polar bonds but
the molecule could be non-polar because
there is no dipole moment
• Bond dipoles can cancel each other out
O
C
O
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Example 3: Dipole moment
• Draw the dipole moment for the following
molecules, are they polar?
HCl
NH3
CHCl3
H2O
SF6
CCl4
CO2
CH2Cl2
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