1.10 The Shapes of Some Simple Molecules

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1.10The Shapes of Some Simple Molecules
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Methane

• tetrahedral geometry
• HCH angle 109.5

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Methane

• tetrahedral geometry
• each HCH angle 109.5

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Valence Shell Electron Pair Repulsions

• The most stable arrangement of groups attached
  to a central atom is the one that has the
  maximum separation of electron pairs(bonded or
  nonbonded).

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Figure 1.9 (a) Water

• bent geometry
• HOH angle 105

H
H

O
..
but notice the tetrahedral arrangement of
electron pairs
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Figure 1.9 (b) Ammonia

• trigonal pyramidal geometry
• HNH angle 107

H
H

N
H
but notice the tetrahedral arrangement of
electron pairs
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Figure 1.9 (c) Boron Trifluoride

• FBF angle 120
• trigonal planar geometry allows for maximum
  separationof three electron pairs

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Multiple Bonds

• Four-electron double bonds and six-electron
  triple bonds are considered to be similar to a
  two-electron single bond in terms of their
  spatialrequirements.

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Figure 1.11 Formaldehyde

• HCH and HCOangles are close to 120
• trigonal planar geometry

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Figure 1.12 Carbon Dioxide

• OCO angle 180
• linear geometry

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1.11Molecular Dipole Moments
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Dipole Moment

• A substance possesses a dipole moment if its
  centers of positive and negative charge do not
  coincide.
• m e x d
• (expressed in Debye units)

not polar
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Dipole Moment

• A substance possesses a dipole moment if its
  centers of positive and negative charge do not
  coincide.
• m e x d
• (expressed in Debye units)

polar
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Molecular Dipole Moments
d
d-
d-

• molecule must have polar bonds
• necessary, but not sufficient
• need to know molecular shape
• because individual bond dipoles can cancel

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Molecular Dipole Moments
Carbon dioxide has no dipole moment m 0 D
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Figure 1.13
Dichloromethane
Carbon tetrachloride
m 0 D
m 1.62 D
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Figure 1.13
Resultant of thesetwo bond dipoles is
Resultant of thesetwo bond dipoles is
m 0 D
Carbon tetrachloride has no dipolemoment
because all of the individualbond dipoles cancel.
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Figure 1.13
Resultant of thesetwo bond dipoles is
Resultant of thesetwo bond dipoles is
m 1.62 D
The individual bond dipoles do notcancel in
dichloromethane it hasa dipole moment.