Honors%20Chemistry

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Honors Chemistry

• Chapter 10 Chemical Bonding II

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10.1 Molecular Geometry

• Study of the shapes of molecules
• Molecules geometry affects properties
• Valence shell
• outermost occupied shell
• Holds electrons involved in bonding
• VSEPR Model
• Valence Shell Electron Pair Repulsion
• Accounts for arrangement of electron pairs around
  a central atom in terms of electrostatic repulsion

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10.1 Molecular Geometry

• AXnEm
• Draw Lewis Structure
• Identify central atom (A)
• Count electron pairs
• X bonding pairs
• E lone pairs

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Electron-group repulsions and the five basic
molecular shapes.
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10.1 Molecular Geometry
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10.1 Molecular Geometry
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10.1 Molecular Geometry
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10.1 Molecular Geometry
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10.1 Molecular Geometry
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10.1 Molecular Geometry
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10.1 Molecular Geometry

• True bond angles deviate from ideal
• Lone pairs are more repulsive than bonding pairs
• Triple bonds gt double bonds gt single bonds in
  terms of repulsion
• Molecules with multiple centers
• Apply VSEPR to each central atom

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10.1 Molecular Geometry

• Find the shapes and bond angles
• AsH3
• CS2
• OF2
• NO3-
• AlCl4-
• I3-
• C2H4

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10.2 Dipole Moments

• Vector describing the polarity of the entire
  molecule
• Symbol for dipole moment is m
• Measured in Debyes
• Depends on bond polarity and geometry
• Examples O2, CO2, H2O, CH4, CCl4, NH3
• Larger molecules become more complex
• cis-dichloroethylene,trans-dichloroethylene

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10.3 Valence Bond Theory

• More complete theory of bonding
• Based on Quantum Mechanics
• Explains bond energies and bond lengths
• Consider overlap of 1s orbitals as two H atoms
  approach each other
• Nucleus-electron attraction forces
• Nucleus-nucleus repulsion forces

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10.3 Valence Bond Theory
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10.4 Hybridization

• Consider compound of C and H
• Carbon has 2s2 sp2 configuration
• Basic QM would predict
• CH2
• 90o bond angles
• Actual compound is CH4, 109.5o angles
• Need to merge s and p orbitals into a new set of
  atomic orbitals (hybrids)

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The sp3 hybrid orbitals in CH4.
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The sp3 hybrid orbitals in NH3.
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The sp3 hybrid orbitals in H2O.
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The sp2 hybrid orbitals in BF3.
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The sp hybrid orbitals in gaseous BeCl2.
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10.4 Hybridization
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10.4 Hybridization

• Procedure for hybridizing atomic orbitals
• 1. Draw Lewis structure
• 2. Use VSEPR to predict the overall geometry of
  the electron pairs
• 3. Deduce the hybridization of the central atom
  from the geometry

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10.5 Double and Triple Bonds

• sp2 and sp hybrids dont use all p orbitals
• Leftover ps can overlap to form bonds
• Sigma bond (s)
• Overlap of s or head-on p orbitals
• e- density between nuclei
• Pi bond (p)
• Sideways overlap of p orbitals
• e- density above and below nuclei

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The s bonds in ethane(C2H6).
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The s and p bonds in ethylene (C2H4).
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The s and p bonds in acetylene (C2H2).
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10.5 Double and Triple Bonds
Electron density and bond order.
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10.5 Double and Triple Bonds
Restricted rotation of p-bonded molecules in
C2H2Cl2.
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10.6 Molecular Orbital Theory

• VB allows e- to stay in atomic orbitals
• This is only an approximation
• Fails to account for some properties of molecules
  (eg, magnetism)
• Reality orbitals are delocalized across the
  entire molecule
• Molecular Orbital Theory based on QM
• Rebuild y for the entire molecule

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An analogy between light waves and atomic wave
functions.
Amplitudes of wave functions added
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Contours and energies of the bonding and
antibonding molecular orbitals (MOs) in H2.
The bonding MO is lower in energy and the
antibonding MO is higher in energy than the AOs
that combined to form them.
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The MO diagram for H2.
Filling molecular orbitals with electrons follows
the same concept as filling atomic orbitals.
s1s
Energy
H2 bond order 1/2(2-0) 1
s1s
MO of H2
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MO diagram for He2 and He2.
s1s
Energy
s1s
MO of He
MO of He2
He2 bond order 0
He2 bond order 1/2
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Bonding in s-block homonuclear diatomic molecules.
Be2
Li2
Be2 bond order 0
Li2 bond order 1
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Contours and energies of s and p MOs through
combinations of 2p atomic orbitals.
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Relative MO energy levels for Period 2
homonuclear diatomic molecules.
without 2s-2p mixing
with 2s-2p mixing
MO energy levels for O2, F2, and Ne2
MO energy levels for B2, C2, and N2
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MO occupancy and molecular properties for B2
through Ne2
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The paramagnetic properties of O2
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The MO diagram for NO
Energy
possible Lewis structures
MO of NO
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10.8 Delocalized Molecular Orbitals

• In larger molecules, bonds are sometimes spread
  over the entire molecule

ozone,O3