Molecular Geometry (Shapes of Molecules)

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Molecular Geometry(Shapes of Molecules)
VSEPR Theory CP Chemistry
http//www.scl.ameslab.gov/MacMolPlt/Surface.JPG
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VSEPR Theory

• Electron groups around the central atom will be
  most stable when they are as far apart as
  possible we call this valence shell electron
  pair repulsion theory
• because electrons are negatively charged, they
  should be most stable when they are separated as
  much as possible
• The resulting geometric arrangement will allow us
  to predict the shapes and bond angles in the
  molecule

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Electrons vs. Molecular Geometry

• The geometry of electron pairs around a central
  atom is called the electron geometry.
• The arrangement of bonded nuclei around a central
  atom forms the molecular geometry.
• Lone pair electrons on a central atom will repel
  other pairs but will not be visible in the
  molecular geometry (no nuclei)
• If there are lone pairs on the central atom the
  electron geometry and the molecular geometry will
  differ.

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Two electron pairs on central atom
Examples CS2, HCN, BeF2
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• 3 electron
• pairs on central atom
• All are in bonds
• Trigonal Planar Shape

Examples SO3, BF3, NO3-, CO32-
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• 3 Electron Pairs
• 2 Bonded
• 1 Un-Bonded

Bent Shape

• Un-bonded electron pair takes up more space and
  repels more.
• The bond angle will change to less than the
  original 120o to about 117o-115o

• Other Examples
• SO2, O3, PbCl2, SnBr2

About 117o
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• Four electron pairs on central atom
• All 4 in bonds

Examples CH4, SiCl4, SO42-, ClO4-
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• 4 Electron Pairs
• 3 bonded
• 1 un-bonded (lone pair)
• Trigonal Pyramidal Shape

Examples NH3, PF3, ClO3. H3O
Bond angles are reduced from 109.5o to 107o due
to extra repulsion by lone pair
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4 Electron Pairs 2 Bonded 2 Un-bonded (lone
pairs) BENT SHAPE
Examples H2O, OF2, SCl2
Bond angles are reduced a little more due to
repulsion To 104.5o
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The steps in determining a molecular shape
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Representing 3-Dimensional Shapes on a
2-Dimensional Surface

• One of the problems with drawing molecules is
  trying to show their dimensionality
• By convention, the central atom is put in the
  plane of the paper
• Put as many other atoms as possible in the same
  plane and indicate with a straight line
• For atoms in front of the plane, use a solid
  wedge
• For atoms behind the plane, use a hashed wedge

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Predicting Molecular Shapes with Two, Three, or
Four Electron Groups
The shape is based upon the tetrahedral
arrangement.
The F-P-F bond angles should be lt109.50 due to
the repulsion of the nonbonding electron pair.
The final shape is trigonal pyramidal.
lt 109.50
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Predicting Molecular Shapes with More Than One
Central Atom
Find the shape of one atom at a time after
writing the Lewis structure.
SOLUTION