Molecular Geometry and Bonding Theories

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Chapter 8

• Molecular Geometryand Bonding Theories

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Molecular Shapes

• The shape of a molecule plays an important role
  in its reactivity.
• By noting the number of bonding and nonbonding
  electron pairs, we can easily predict the shape
  of the molecule.

Figure 8.2
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CH4 , C , H2O (SO4)-2 , (SiO4)-4
(PO4)-3 , CCl2F2
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What Determines the Shape of a Molecule?

• Simply put, electron pairs, whether they are
  bonding or nonbonding, repel each other.
• By assuming the electron pairs are placed as far
  as possible from each other, we can predict the
  shape of the molecule.

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Valence Shell Electron Pair Repulsion Theory
(VSEPR)

• The best arrangement of a given number of
  electron domains is the one that minimizes the
  repulsions among them.

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Electron Domains

• We can refer to the electron pairs as electron
  domains.
• In a double or triple bond, all electrons shared
  between those two atoms are on the same side of
  the central atom. Therefore, they count as one
  electron domain.

This molecule has four electron domains.
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Electron-Domain Geometries

• These are the electron-domain geometries for two
  through six electron domains around a central
  atom.

Table 8.1
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Electron-Domain Geometries

• All one must do is count the number of electron
  domains in the Lewis structure.
• The geometry will be that which corresponds to
  that number of electron domains.

Figure 8.6
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Molecular Geometries

• The electron-domain geometry is often not the
  shape of the molecule, however.
• The molecular geometry is defined by the
  positions of only the atoms in the molecules, not
  the nonbonding pairs.

Figure 8.6
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Electron Domain Linear

• In this domain, there is only one molecular
  geometry linear.
• NOTE If there are only two atoms in the
  molecule, the molecule will be linear no matter
  what the electron domain geometry is.

Table 8.2
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Molecular Geometries

• Within each electron domain structure,
• there might be more than one molecular geometry.
• Given these examples, try and draw the electron
  domain geometry for OH- ,
• The hydroxide ion.
• Once you try this,
• look at the notes for more chemical consequences
  of these simple geometries.

Table 8.2
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Electron Domain Trigonal Planar

• There are two molecular geometries
• Trigonal planar, if all the electron domains are
  bonding
• Bent, if one of the domains is a nonbonding pair.

Table 8.2
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Electron Domain Tetrahedral

• There are three molecular geometries
• Tetrahedral, if all are bonding pairs
• Trigonal pyramidal, if one is a nonbonding pair
• Bent, if there are two nonbonding pairs

Table 8.2
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Electron DomainTrigonal Bipyramidal

• There are two distinct positions in this
  geometry
• Axial (Apical if degenerate)
• Equatorial
• Name and draw the 4 possible degenerate
  geometries.

Figure 8.8
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Electron Domain Trigonal Bipyramidal

• Lower-energy conformations result from having
  nonbonding electron pairs in equatorial, rather
  than axial, positions in this geometry.

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Electron Domain Trigonal Bipyramidal

• There are four distinct molecular geometries in
• this domain
• Trigonal bipyramidal
• Seesaw
• T-shaped
• Linear

Table 8.3
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Electron Domain Octahedral

• All positions are equivalent in the octahedral
  domain. There are three molecular geometries
• Octahedral
• Square pyramidal
• Square planar

Table 8.3
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The Effect of Nonbonding Electrons and Multiple
Bonds on Bond Angles

• Nonbonding pairs are physically larger than
  bonding pairs (all charge and virtually no mass
  to constrain them).
• Therefore, their repulsions are greater this
  tends to decrease bond angles in a molecule.

Figure 8.7
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The Effect of Nonbonding Electrons and Multiple
Bonds on Bond Angles

• Double and triple bonds place greater electron
  density on one side of the central atom than do
  single bonds.
• Therefore, they also affect bond angles.
• For hazards and gas warefare read notes.

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Shapes of Larger Molecules

• In larger molecules, it makes more sense to talk
  about the geometry of a particular atom rather
  than the geometry of the molecule as a whole.

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Molecular Shape and Molecular Polarity

• If a molecule possesses polar bonds, it does not
  mean the molecule as a whole will be polar.

Figure 8.11
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Molecular Shape and Molecular Polarity

• By adding the individual bond dipoles, one can
  determine the overall dipole moment for the
  molecule.

Figure 8.12
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Molecules Containing Polar Bonds
Figure 8.13
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F12
Ozone-depleting gas trends
F22
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Chapter 8 End of Part 1

• Molecular Shapes
• Electron Domain Shapes