General Chemistry

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Molecular Shapes
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Molecular Shapes
In order to predict molecular shape, we assume
the valence electrons repel each other.
Therefore, the molecule adopts whichever 3D
geometry minimized this repulsion. We call this
process Valence Shell Electron Pair Repulsion
(VSEPR) theory.
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The VSEPR Model common Mol. Geo.
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Electron-Domain Geo.
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The VSEPR Model
Predicting Molecular Geometries
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The VSEPR Model
Predicting Molecular Geometries
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The VSEPR Model
Predicting Molecular Geometries
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The VSEPR Model
Molecules with Expanded Valence Shells
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The VSEPR Model
Molecules with Expanded Valence Shells
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The VSEPR Model

• We determine the electron domain geometry by
  looking at electrons around the central atom.
• We name the molecular geometry by the positions
  of atoms.
• We ignore lone pairs in the molecular geometry.

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The VSEPR Model
The Effect of Nonbonding Electrons and Multiple
Bonds on Bond Angles By experiment, the H-X-H
bond angle decreases on moving from C to N to
O Since electrons in a bond are attracted by
two nuclei, they do not repel as much as lone
pairs. Therefore, the bond angle decreases as the
number of lone pairs increase.
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The VSEPR Model
The Effect of Nonbonding Electrons and Multiple
Bonds on Bond Angles Similarly, electrons in
multiple bonds repel more than electrons in
single bonds.
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The VSEPR Model
Molecules with More than One Central Atom In
acetic acid, CH3COOH, there are three central
atoms. We assign the geometry about each central
atom separately.
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Polarity of Molecules
Polar molecules interact with electric fields. If
the centers of negative and positive charge do
not coincide, then the molecule is polar.
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Polarity of Molecules
Dipole Moments of Polyatomic Molecules Example
in CO2, each C-O dipole is canceled because the
molecule is linear. In H2O, the H-O dipoles do
not cancel because the molecule is bent.
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Polarity of Molecules
Dipole Moments of Polyatomic Molecules
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Covalent Bonding and Orbital Overlap

• Lewis structures and VSEPR do not explain why a
  bond forms.
• How do we account for shape in terms of quantum
  mechanics?
• What are the orbitals that are involved in
  bonding?
• We use Valence Bond Theory
• Bonds form when orbitals on atoms overlap.
• There are two electrons of opposite spin in the
  orbital overlap.

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Covalent Bonding and Orbital Overlap
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Covalent Bonding and Orbital Overlap
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Hybrid Orbitals

• sp Hybrid Orbitals
• Consider the BeF2 molecule (experimentally known
  to exist)
• Be has a 1s22s2 electron configuration.
• There is no unpaired electron available for
  bonding.
• We conclude that the atomic orbitals are not
  adequate to describe orbitals in molecules.
• We know that the F-Be-F bond angle is 180? (VSEPR
  theory).
• We also know that one electron from Be is shared
  with each one of the unpaired electrons from F.

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Formation of sp Hybrid Orbital
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Formation of sp2 Orbitals
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Formation of sp3 Orbitals
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Bonding in H2O
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Hybridization in Ethylene
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Table 9.4p 366
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Hybrid Orbitals

• Summary
• To assign hybridization
• draw a Lewis structure
• assign the electron pair geometry using VSEPR
  theory
• from the electron pair geometry, determine the
  hybridization and
• name the molecular geometry by the positions of
  the atoms.

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Pi Bond Formation in Ethylene
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Triple Bond in Acetylene
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Bonding in Benzene
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Orbitals of Benzene
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Molecular Orbitals

• Some aspects of bonding are not explained by
  Lewis structures, VSEPR theory and hybridization.
  (E.g. why does O2 interact with a magnetic
  field? Why are some molecules colored?)
• For these molecules, we use Molecular Orbital
  (MO) Theory.
• Just as electrons in atoms are found in atomic
  orbitals, electrons in molecules are found in
  molecular orbitals.
• Molecular orbitals
• each contain a maximum of two electrons
• have definite energies
• can be visualized with contour diagrams
• are associated with an entire molecule.

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Molecular Orbitals
The Hydrogen Molecule
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MO Electron Configurations
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