Transcript Chem 1201 - LSU Department of Chemistry

Watkins
Chapter 9
Chapter 9
Molecular (Bond) Geometry
VSEPR
Polarity
Orbital Overlap
Hybrid Orbitals
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Chapter 9
MOLECULAR SHAPES
CCl4 has a specific 3-D Atomic Arrangement
Cl
The shape of this molecule is
derived from the Lewis Diagram
Cl C
The molecule has specific
Cl
Bond Lengths and Bond Angles
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Cl
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Chapter 9
MOLECULAR SHAPES
CCl4 has a specific 3-D Atomic Arrangement
This shape is called tetrahedral because
the peripheral atoms are arranged at
the corners of a tetrahedron.
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Cl
Cl
C
Cl
Cl
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Chapter 9
MOLECULAR SHAPES
Some of the other basic molecular shapes are shown below:
2 PAs 3 PAs 4 PAs 5 PAs 6 PAs
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Chapter 9
MOLECULAR SHAPES
Molecular shape is determined by VSEPR
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Chapter 9
VSEPR
Valence Shell Electron Pair Repulsion
Draw the Lewis diagram showing all LPs & BPs,
then count the electron domains around each atom.
One Electron Domain is
– a lone pair or ...
– a covalent bond (of any bond order)
Arrange the electron domains about each atom so that
Each electron domain is as far away from the other
electron domains as possible, because ...
The electron domains repel one another
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Chapter 9
VSEPR - Electron Domain Geometry
Table 9.1
Electron Domains
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Electron Domain
Geometry
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Chapter 9
VSEPR - Electron Domain Geometry
Table 9.1
Electron Domain
Geometry
Electron Domains
2 axial
3 equatorial
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Chapter 9
VSEPR - Electron Domain Geometry
Summary of Five EDGs
Trigonal
Planar
Linear
Tetrahedral
Trigonal
Bipyramidal
Octahedral
All Molecular (Bond) Geometries are
derived from these five EDGs
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Chapter 9
VSEPR - Bonding Geometry
Table 9.2
Bonding
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Chapter 9
VSEPR - Bonding Geometry
Table 9.2
Bonding
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Chapter 9
VSEPR - Bonding Geometry
Table 9.3
Axial
Equatorial
Bonding
*
1. Axial positions are
occupied by the most
electronegative
atoms.
2. Lone Pairs have zero
electronegativity.
Lone pairs always
occupy equatorial
positions.
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Chapter 9
VSEPR - Bonding Geometry
Table 9.3
Bonding
6
All six vertices of
an octahedron
are identical.
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Chapter 9
VSEPR - Bonding Geometry
Homepage > Extra > VSEPR
#dom #BP #LP Form
2 2
0 AB2
1
1 XAB
3 3
0 AB3
2
1 XAB2
1
2 X2AB
4 4
0 AB4
3
1 XAB3
2
2 X2AB2
1
3 X3AB
Geom
linear
linear
tri plan
bent
linear
tetr
tri pyr
bent
linear
#dom #BP #LP Form
5 5
0 AB5
4
1 XAB4
3
2 X2AB3
2
3 X3AB2
1
4 X4AB
6 6
0 AB6
5
1 XAB5
4
2 X2AB4
3
3 X3AB3
2
4 X4AB2
1
5 X5AB
A = Central Atom
B = Peripheral Atom
X = Lone Pair
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Geom
tri bipyr
see-saw
T
linear
linear
oct
sq pyr
sq plan
T
linear
linear
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Chapter 9
VSEPR - Bonding Geometry
Complex Molecules
The geometry of each Central Atom is determined by VSEPR
H
H
H
C
C
H
H
EDG = Tetrahedral
BG = Bent
O
H
EDG = Tetrahedral
BG = Tetrahedral
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Chapter 9
Bond Polarity
Chapter 8, pg. 314
For a bonded pair of atoms A—B, with
electronegativities cA and cB
Bond polarity depends on the difference in c
Dc = |cA – cB|
If Dc = 0, bond is non-polar
If Dc > 0, bond is polar
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Chapter 9
Bond Polarity
Chapter 8, pg. 314
For a bonded pair of atoms A—B, with
electronegativities cA and cB
Bond polarity depends on the difference in c
Dc = |cA – cB|
Most electronegative atom is Negative
Least electronegative atom is Positive
d+A—BdChemistry 1421
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(dipole)
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Chapter 9
Molecule Polarity
For a diatomic molecule (only one bond)
Molecule Polarity = Bond Polarity
d+H—Fd-
Dipole moment
For any molecule with two or more bonds,
Molecule Polarity = Vector Sum of bond dipoles
Vector Sum is like tug-o-war
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Chapter 9
Molecule Polarity
tug-o-war
Overall dipole moment
>0
Overall dipole moment
=0
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Chapter 9
Molecule Polarity
Fig 9.13 tug-o-war
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VSEPR - Molecular Geometry
#dom #BP #LP Form
2 2
0 AB2
1
1 XAB
3 3
0 AB3
2
1 XAB2
1
2 X2AB
4 4
0 AB4
3
1 XAB3
2
2 X2AB2
1
3 X3AB
Geom
linear
linear
tri plan
bent
linear
tetr
tri pyr
bent
linear
#dom #BP #LP Form
5 5
0 AB5
4
1 XAB4
3
2 X2AB3
2
3 X3AB2
1
4 X4AB
6 6
0 AB6
5
1 XAB5
4
2 X2AB4
3
3 X3AB3
2
4 X4AB2
1
5 X5AB
A = Central Atom
B = Peripheral Atom
X = Lone Pair
Chapter 9
Geom
tri bipyr
see-saw
T
linear
linear
oct
sq pyr
sq plan
T
linear
linear
Nonpolar (if all B atoms are identical)
Polar
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Chapter 9
MOLECULAR SHAPES
Electron Domains determined by the
Lewis Diagram
Domain Geometry predicted by VSEPR
Covalent Bonds formed by Orbital Overlap
Bonding Domain (Molecular) Geometry
predicted by VSEPR
described by Hybridization
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Chapter 9
Orbital Overlap
In H2, the shared-pair covalent bond is formed by
overlap of the two atomic 1s orbitals.
The “lens” shaped region of
overlap lies along the internuclear
line.
Covalent bonds with overlap along
the internuclear line are called
"sigma" (s) bonds .
The covalent bond in H2 is
symbolized "H-H s-s s".
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1s
H-H s-s s
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Chapter 9
Orbital Overlap
Hydrogen must use the pure 1s
atomic orbital for bonding because
it is the only valence shell orbital
available to this atom.
All other bonding atoms have p
orbitals in their valence shells.
All other atoms use hybrid atomic
orbitals to form s-bonds.
1s
H-H s-s s
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Chapter 9
Hybrid Orbital Geometry
Hybrid orbitals form because the geometry of the
pure atomic orbitals is not consistent with the
observed bonding geometries of central atoms.
But hybrid orbitals, which are combinations
of pure atomic orbitals, are consistent with
observed bonding geometries.
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Chapter 9
Hybrid Orbital Geometry
For example, CH4 (methane) has a tetrahedral
shape with bond angles of 109.5o, but s and p
orbitals would produce only with 90o angles.
H
H
C
H
H
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Chapter 9
Hybrid Orbital Geometry
The four atomic orbitals can be combined to
form four hybrid atomic orbitals.
The four hybrid
atomic orbitals
are tetrahedral
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All 4 orbtials are
equivalent and are
called "sp3" hybrid
orbitals.
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Chapter 9
Hybrid Orbital Geometry
Each 1s atomic orbital of an H-atom overlaps
with a C sp3 hybrid orbital to form a
"H-C s-sp3 s" bond.
Lens-shaped
s overlap
H
C
H
H
H
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Chapter 9
Hybrid Orbital Geometry
Another example is ethylene, C2H4, which has bond
angles of 120o, not 90o.
H
H
C
C
H
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H
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Chapter 9
Hybrid Orbital Geometry
If the s orbital and two of the p orbitals (px and py)
in a carbon atom are combined, they produce
three hybrid orbitals all at 120o
These three are
called sp2
hybrid orbitals
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The pz orbital
remains
unaltered.
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Chapter 9
Hybrid Orbital Geometry
The sp2 hybrid orbitals on each C atom overlap
with each other and with H-1s orbitals to form
five shared-pair covalent sigma bonds.
What happens
to the two pz
orbitals?
C-C sp2-sp2 s
H-C s-sp2 s
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Chapter 9
Hybrid Orbital Geometry
Two parallel p orbitals can
overlap side by side to form
one covalent bond.
The book draws p-orbitals
skinny, but they are really fat.
The overlap region is above
and below the internuclear
line. This type of shared
pair covalent bond is called
a pi (p) bond.
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Chapter 9
Hybrid Orbital Geometry
This type of shared pair
covalent bond is called a
pi (p) bond.
The two "banana-lobes" above and below the
internuclear line both belong to one p bond,
and only one pair of electrons inhabits both
lobes simultaneously.
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Chapter 9
Hybrid Orbital Geometry
So the ethylene molecule contains three
kinds of covalent bonds:
one C-C sp2-sp2 s bond
four H-C s-sp2 s bonds
one C-C p-p p bond
H
H
C
H
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C
H
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Chapter 9
Hybrid Orbital Geometry
Electron Domain Geometry,
predicted by L.D. + VSEPR;
= Hybrid Orbital Geometry,
the spatial arrangement of hybrid
orbitals around a central atom;
= s Bond & LP Geometry,
the spatial arrangment of s-bonds
around a central atom.
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Chapter 9
Hybrid Orbital Geometry
Table 9.4
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Chapter 9
Hybrid Orbital Geometry
Table 9.4
Note: sp3d and sp3d2 geometries are only available if the central
atom is in the 3rd row (or higher) of the periodic table, when d
orbitals become available in the valence shell.
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Chapter 9
Analyzing a Molecule
NH3
Lewis Structure
Electron Domain Geometry
(tetrahedral)
Hybrid Orbital Geometry
(tetrahedral, sp3)
3 H-N s-sp3 s bonds
1 lone pair
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Molecular Geometry
(trigonal pyramidal)
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