VSEPR MODEL

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VSEPR MODEL Valence Shell Electron Pair Repulsion
Model A model for predicting the shapes of
molecules and ions in which valence shell
electron pairs are arranged about each atom so
that electron pair repulsion is minimized.
ELECTRONIC GEOMETRY The general shape of a
molecule determined by the number of electron
pairs around the central atom occupying different
quadrants. Gives starting point for bond
angle. MOLECULAR GEOMETRY The general shape of a
molecule determined by the relative positions of
the atomic nuclei. The nonbonding electron pairs
modifiy the geometry.
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VSEPR MODEL I. Draw the Lewis dot
structure. II. Determine the electronic
geometry by counting the number of pairs of
electrons around the central atom occupying
different quadrants (top, bottom, left, right).
This geometry gives the initial bond
angle. Pairs of e- geometry bond
angle 2 linear
180o 3
trigonal planar 120o
4 tetrahderal
109.5o
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VSEPR MODEL III. Next, using the electronic
geometry, determine the number of bonding and
nonbonding electron pairs then arrange the
electron pairs as far apart as possible. ___
nonbonding pairs require more space than bonding
pairs. ___ multiple bonds require more space
than single bonds. IV. The direction in space
of the bonding pairs give the molecular geometry
modified by the position of the nonbonding
pairs.
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Table describing Molecular GeometryVSPER Theory

• Number of electronic bonding nonbonding
  molecular
• e- pairs geometry
  e- pairs e- pairs geometry
  
• 2 linear 2 0 linear
• 3 trigonal planar 3
  0 trigonal planar
• 3 trigonal planar 2
  1 bent
• 4 tetrahedral 4
  0 tetrahedral
• 4 tetrahdral 3
  1 trigonal pyramidal
• 4 tetrahedral 2
  2 bent

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• Predict the Molecular Geometry for the
  following molecules.
• 1. H2O
• There are 4 pairs of electrons around the central
  atom so the electronic geometry is tetrahedral
  with a bond angle of 109.5o. Since 2 of the
  pairs of electrons are bonding and the other two
  pairs are nonbonding, the electronic geometry
  has been modified to a molecular geometry of
  BENT.
• 2. CO2
• There are four pairs of electrons in two
  different quadrents (left right) so the
  molecular geometry is LINEAR with a bond angle of
  180o.
• . . . .
• O C O

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• Predict the Molecular Geometry for the
  following molecules.
• 3. BH3
• There are 3 pairs of electrons around the central
  atom so the electronic geometry is trigonal
  planar with a bond angle of 120o. Since all
  three pairs of electrons are bonding, the
  electronic geometry has not been modified.
• 4. NH3
• There are 4 pairs of electrons around the central
  atom so the electronic geometry is tetrahedral
  with a bond angle of 109.5o. Since 3 of the
  pairs of electrons are bonding and the other pair
  is nonbonding, the electronic geometry has been
  modified to a molecular geometry of TRIGONAL
  PYRAMIDAL.

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Practice Problems

• Draw the Lewic structure for the following, then
  predict both the electronic molecular geometry.
  Give the approximate bond angle. (See instructor
  for answers)
• a) GeH2 b) AsF3 c) AlF3
• d) SO2 e) SO3 f) SO32-
• g) SiF4 h) C2H4 I) Cl2O

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Group Study Problems

• Draw the Lewic structure for the following,
  then predict both the electronic molecular
  geometry. Give the approximate bond angle.
• a) H2S b) PH3 c) CH2O
• d) NO2- e) GaH3 f) CBr4
• g) CH2FCl h) C2H2 I) O3