Covalent Bonding: Molecular Geometry

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Covalent BondingMolecular Geometry
Hybridization of Atomic Orbitals Molecular
Orbitals
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Valence shell electron pair repulsion (VSEPR)
model
Predict the geometry of the molecule from the
electrostatic repulsions between the electron
(bonding and nonbonding) pairs.
AB2
2
0
linear
linear
AB4
4
0
tetrahedral
tetrahedral
AB6
6
0
octahedral
octahedral
10.1
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VSEPR
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VSEPR
BF3
CH4
PCl5
SF6
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Effects of Lone Pairs
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VSEPR
trigonal planar
trigonal planar
AB3
3
0
AB2E
2
1
8
VSEPR
AB4
4
0
tetrahedral
tetrahedral
AB3E
3
1
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VSEPR
AB4
4
0
tetrahedral
tetrahedral
AB2E2
2
2
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VSEPR
trigonal bipyramidal
trigonal bipyramidal
AB5
5
0
AB4E
4
1
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VSEPR
trigonal bipyramidal
trigonal bipyramidal
AB5
5
0
AB3E2
3
2
12
VSEPR
trigonal bipyramidal
trigonal bipyramidal
AB5
5
0
AB2E3
2
3
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VSEPR
AB5E
5
1
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VSEPR
AB4E2
4
2
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Predicting Molecular Geometry

1. Draw Lewis structure for molecule.
2. Count number of lone pairs on the central atom
   and number of atoms bonded to the central atom.
3. Use VSEPR to predict the geometry of the molecule.

AB4E
AB2E
distorted tetrahedron
bent
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Dipole Moments and Polar Molecules
electron rich region
electron poor region
m Q x r
Q is the charge
r is the distance between charges
1 D 3.36 x 10-30 C m
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dipole moment polar molecule
dipole moment polar molecule
no dipole moment nonpolar molecule
no dipole moment nonpolar molecule
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Chemistry In Action Microwave Ovens
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Sharing of two electrons between the two atoms.
Valence bond theory bonds are formed by sharing
of e- from overlapping atomic orbitals.
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Change in electron density as two hydrogen atoms
approach each other.
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Valence Bond Theory and NH3
N 1s22s22p3
3 H 1s1
If use the 3 2p orbitals predict 900
Actual H-N-H bond angle is 107.30
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Hybridization mixing of two or more atomic
orbitals to form a new set of hybrid orbitals.

• Mix at least 2 nonequivalent atomic orbitals
  (e.g. s and p). Hybrid orbitals have very
  different shape from original atomic orbitals.
• Number of hybrid orbitals is equal to number of
  pure atomic orbitals used in the hybridization
  process.
• Covalent bonds are formed by
• Overlap of hybrid orbitals with atomic orbitals
• Overlap of hybrid orbitals with other hybrid
  orbitals

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Bonding in Methane
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Formation of sp3 Hybrid Orbitals
Fig. 10.7
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Formation of sp3 Hybrid Orbitals
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Formation of a CH4 Molecule
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Formation of a NH3 Molecule
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Stylized Drawing of Valence Bond Theory
NH3
CH4
Sigma bond (s) electron density between the 2
atoms
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Formation of sp2 Hybrid Orbitals
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Formation of sp2 Hybrid Orbitals
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Formation of sp2 Hybrid Orbitals
2pz orbital is perpendicular to the plane of
hybridized orbitals
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sp2 Hybridization of a C atom
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Bonding in Ethylene C2H4
H
H
C
C
H
H
Sigma bond (s) electron density between the 2
atoms
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Bonding in Ethylene C2H4
Pi bond (p) electron density above and below
plane of nuclei of the bonding atoms
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Bonding in Ethylene C2H4
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Formation of sp Hybrid Orbitals
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Formation of sp Hybrid Orbitals
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Formation of sp Hybrid Orbitals
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Bonding in acetylene C2H2
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Count the number of lone pairs AND the number of
atoms bonded to the central atom
of Lone Pairs of Bonded Atoms
Hybridization
Examples
2
sp
BeCl2
3
sp2
BF3
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sp3
CH4, NH3, H2O
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sp3d
PCl5
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sp3d2
SF6
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Sigma (s) and Pi Bonds (p)
1 sigma bond
Single bond
1 sigma bond and 1 pi bond
Double bond
Triple bond
1 sigma bond and 2 pi bonds
s bonds 6
1 7
p bonds 1
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Drawback of Valence Bond Theory
No unpaired e-
Should be diamagnetic
Molecular orbital theory bonds are formed from
interaction of atomic orbitals to form molecular
orbitals.
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An analogy between light waves and atomic wave
functions
Amplitudes of wave functions added
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Energy levels of bonding and antibonding
molecular orbitals in hydrogen (H2)
A bonding molecular orbital has lower energy and
greater stability than the atomic orbitals from
which it was formed.
An antibonding molecular orbital has higher
energy and lower stability than the atomic
orbitals from which it was formed.
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Energy levels of bonding and antibonding
molecular orbitals in boron (B2)
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Second-Period Homonuclear Diatomic Molecules
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Molecular Orbital (MO) Configurations

• The number of molecular orbitals (MOs) formed is
  always equal to the number of atomic orbitals
  combined.
• The more stable the bonding MO, the less stable
  the corresponding antibonding MO.
• The filling of MOs proceeds from low to high
  energies.
• Each MO can accommodate up to two electrons.
• Use Hunds rule when adding electrons to MOs of
  the same energy.
• The number of electrons in the MOs is equal to
  the sum of all the electrons on the bonding atoms.

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Bond Order
Number of electrons in bonding MOs
Number of electrons in antibonding MOs
(
)
-
bond order
bond order
1
½
0
½
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MO for 2nd Period Homonuclear Diatomic Molecules
MO theory predicts that O2 is paramagnetic!
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Molecules with Resonance Structures
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Delocalized p Molecular Orbitals
Delocalized molecular orbitals are not confined
between two adjacent bonding atoms, but actually
extend over three or more atoms.
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Electron density above and below the plane of the
benzene molecule.
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Acknowledgment
Some images, animation, and material have been
taken from the following sources Chemistry,
Zumdahl, Steven S. Zumdahl, Susan A. Houghton
Mifflin Co., 6th Ed., 2003 supplements for the
instructor General Chemistry The Essential
Concepts, Chang, Raymon McGraw-Hill Co. Inc.,
4th Ed., 2005 supplements for the
instructor Principles of General Chemistry,
Silberberg, Martin McGraw-Hill Co. Inc., 1st
Ed., 2006 supplements for the instructor