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Molecular Geometry and Bonding Theories

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Title: Molecular Geometry and Bonding Theories


1
Chapter 8
  • Molecular Geometry and Bonding Theories

2
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
3
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4
CH4 , C , H2O (SO4)-2 , (SiO4)-4
(PO4)-3 , CCl2F2
5
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6
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.

7
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.

8
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9
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.
10
Electron-Domain Geometries
  • These are the electron-domain geometries for two
    through six electron domains around a central
    atom.

Table 8.1
11
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
12
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
13
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
14
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
15
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
16
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
17
Electron Domain Trigonal 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
18
Electron Domain Trigonal Bipyramidal
  • Lower-energy conformations result from having
    nonbonding electron pairs in equatorial, rather
    than axial, positions in this geometry.

19
Electron Domain Trigonal Bipyramidal
  • There are four distinct molecular geometries in
  • this domain
  • Trigonal bipyramidal
  • Seesaw
  • T-shaped
  • Linear

Table 8.3
20
Electron Domain Octahedral
  • All positions are equivalent in the octahedral
    domain. There are three molecular geometries
  • Octahedral
  • Square pyramidal
  • Square planar

Table 8.3
21
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
22
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.

23
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.

24
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25
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
26
Molecular Shape and Molecular Polarity
  • By adding the individual bond dipoles, one can
    determine the overall dipole moment for the
    molecule.

Figure 8.12
27
Molecules Containing Polar Bonds
Figure 8.13
28
F12
Ozone-depleting gas trends
F22
29
Chapter 8 End of Part 1
  • Molecular Shapes
  • Electron Domain Shapes
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