Molecular Geometry and Bonding Theories
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Molecular Geometry and Bonding Theories
•The overall shape of a molecule is determined by its bond angles
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Common Bonding Geometries
AB3E1
AB2
AB2E1
AB3
AB4
•Atoms (B) which are bonded to a central atom (A): ABn
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Valence Shell Electron Pair Repulsion Theory
VSEPR: the best arrangement of a given number of electron pairs is the
one that minimizes the repulsions among them.
Nonbonding pairs
Bonding pairs
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Valence Shell Electron Pair Repulsion Theory
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Valence Shell Electron Pair Repulsion Theory
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VSEPR:Four or Fewer Valence-Shell Electron
Pairs Around a Central Atom
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VSEPR:Four or Fewer Valence-Shell Electron
Pairs Around a Central Atom
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VSEPR:The Effect of Nonbonding Electrons
and Multiple Bonds on Bond Angles
Nonbonding electron pairs exert greater repulsive forces on adjacent electron pairs
and thus tend to compress the angles between the bonding pairs
Nonbonding electron pairs
109. 5°
Methane (CH4)
107°
Ammonia (NH3)
2 Nonbonding electron pairs
104 .°5
Water (H2O)
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VSEPR:The Effect of Nonbonding Electrons
and Multiple Bonds on Bond Angles
Multiple bonds, like nonbonding electron pairs, exert a greater repulsive force on
adjacent electron pairs than do single bonds.
122°
116 °
122 °
Formaldehyde (H2CO)
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VSEPR: Geometries of Molecules with Expanded
Valence Shells
When the central atom of an ABn molecule is from the third period of the periodic table
and beyond, the atom may have more than four electron pairs around it.
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VSEPR: Geometries of Molecules with Expanded
Valence Shells
Note: because nonbonding electron pairs exert larger repulsion than bond pairs,
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they always occupy the equatorial positions in AB5 molecules
VSEPR: Geometries of Molecules with Expanded
Valence Shells
Note: the placement of a single nonbonding electron pair in an AB6 molecule is
arbitrary. The placement of two nonbonding pairs must be on opposite sides of 13
the octahedron.
VSEPR: Molecules with No Single Central Atom
Tetrahedral
Trigonal Planar
Bent
H
H
O
C
C
..
..O
H
H
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Now Wait Just a Dipole Moment!
 = Qr, where  is the dipole moment in
‘Debyes (3.33 x 10-30 C-m/D)’, Q is the
charge at each end of the molecule and
r is the distance between the charges
Given that the dipole moment for HCl
( ) = 1.03 D how much separation of
charge exists between the H and Cl
atoms.
 = Qr
1.03D ( 3.33 x 10-30 C-m/D) = Q (1.36 A)
(1x 10-10m/A)
Q = 2.52 x 1020C
%Q = (2.52 x 1020C/1.6 x 10-19C)x 100
= 15.8
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Dipole Moment: The Polarity of Polyatomic
Molecules
The polarity of a molecule is dependent upon both the polarities of
the bonds and the geometry of the molecule
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Dipole Moment: The Polarity of Polyatomic
Molecules
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Covalent Bonding and Orbital Overlap:
Valence Bond Theory
Valence Bond Theory unifies Lewis’ notion of electron-pair bonds and the
quantum notion of atomic orbitals.
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Covalent Bonding and Orbital Overlap:
Valence Bond Theory
-bonds: results from the overlapping of two s-orbitals, one from each atom.
-bonds appear to
be weaker than
-bonds!
-bonds
-bonds
Formaldehyde (H2CO)
-bonds: result from the overlapping of two p-orbitals, one from each atom.
These covalent bonds are constructed from overlapped regions above and
below the internuclear axis
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:
:
Covalent Bonding and Orbital Overlap:
Hybrid Orbitals
:F
:
2p5
:
1s2 2s2
Be F :
1s2 2s2
2p5
1s2 2s2
In order for an overlap to occur Beryllium must provide unpaired
electrons. This is accomplished by promoting one of the 2S electrons
1s2 2s1
2p1
In order for each of the bonds to be identical, the 2s1 and 2p1 orbitals
become hybridized
sp
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Covalent Bonding and Orbital Overlap:
Hybrid Orbitals (sp, BeF2)
Large lobes of sp hybrid orbitals
Be
Fluorine 2p orbitals
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Covalent Bonding and Orbital Overlap:
Hybrid Orbitals (sp2, BF3)
B
1s2
2s 2
2p1
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Covalent Bonding and Orbital Overlap:
Hybrid Orbitals (sp2, BF3)
In order for an overlap to occur Boron must provide 3 unpaired
electrons. This is accomplished by promoting one of the 2S electrons
1s2 2s1
2p1
In order for each of the bonds to be identical, the 2s1 and 2p1 orbitals
become hybridized
sp2
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Covalent Bonding and Orbital Overlap:
Hybrid Orbitals (sp2, BF3)
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Covalent Bonding and Orbital Overlap:
Hybrid Orbitals (sp3, CH4)
1s2 2s2
109. 5°
2p1
promote
Methane (CH4)
1s2 2s1
2p3
hybridize
sp3
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Covalent Bonding and Orbital Overlap:
Hybrid Orbitals (sp3, CH4)
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Covalent Bonding and Orbital Overlap:
Hybrid Orbitals (sp3d, PF5)
3s2
3p3
3d
promote
Phosphorous Pentafluoride (PF5)
3s2
3p3
•Atoms in the third period and beyond can
use their d orbitals
sp3d1
3d
hybridize
3d
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Geometrical Arrangements
Characteristic of Hybrid Orbitals
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Geometrical Arrangements
Characteristic of Hybrid Orbitals
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Hybrid Orbitals and Multiple Bonds:
Getting a Piece of the ,
Consider the molecular geometry of ethylene:
1s2 2s2
2p2
promote
1s2 2s1
2p3
hybridize
sp2
•Note that the bond angles are probably trigonal planar. This suggests an sp2
configuration with one electron remaining in an unhybridized 2p orbital
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Hybrid Orbitals and Multiple Bonds:
Getting a Piece of the ,
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Hybrid Orbitals and Multiple Bonds:
Getting a Piece of the ,
Consider the molecular geometry of acetylene
1s2 2s2
2p2
promote
1s2 2s1
2p3
hybridize
sp
•Note that the bond angles are probably linear. This suggests an sp configuration 32
with each of the two remaining electrons located in unhybridized 2p orbital
Hybrid Orbitals and Multiple Bonds:
Getting a Piece of the ,
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What about SO2
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Hybrid Orbitals and Multiple Bonds:
Delocalized Bonding
In noncyclic compounds, bonding electrons are localized
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Hybrid Orbitals and Multiple Bonds:
Delocalized Bonding
Cyclic compounds resonate, in these cases the  bonds are
delocalized
This molecules are particularly stable and are described as
aromatics
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Molecular Orbitals: Its Quantum Time!
antibonding orbitals
bonding orbitals
Contour Representations
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H2, He2
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O2, F2, O2-1
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Bond order = bonding
electrons – antibonding
electrons divided by 2
paramagnetic- magnetic field
caused by unpaired electrons
diamagnetic- weakly repelled by
magnetic field, caused by all
electrons being paired.
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B2, C2, N2, NO
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