Warm Up

1
Warm Up

• Draw Lewis structures for CCl4 and CH2O

2
Molecular Geometry

• The SHAPE of MOLECULES

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Who cares about molecular shape?

• The shape of molecules determine how they
  interact with other molecules. Especially
  important in biochemistry. For example enzymes
  ride along DNA or RNA until they chemically
  fit then they begin working.

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Molecular Shape

• Lewis structures tell us how atoms are bonded
  together, but tell us NOTHING about how the atoms
  are arranged in 3-D.
• Could you have predicted the arrangement of atoms
  shown below from just seeing the Lewis Structure??

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So how do we find the shape of a molecule?
VSEPR Theory
alence hell lectron air epulsion
V S E P R
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Main Premise of VSEPR Theory

• Molecules will adopt a shape that is lowest in
  energy
• A low energy shape is one that minimizes the
  valence shell electron pair repulsion

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Huh? In English PLEASE

• Atoms in a molecule try to spread out from one
  another as much as possible
• to reduce the like charge repulsion between
  their outer electrons.

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methane, CH4
You might think this is the farthest that the
hydrogens can get away from each other
90 angles
109.5 angles
But if you think in 3 dimensions, this shape
actually causes less repulsion between the
bonding pairs of electrons
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5 Main Shapes

• This is three of them (others come from these 3)

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Here are the STEPS fordetermining molecular
geometry

• 1. Draw a Lewis structure
• 2. Count the of atoms connected and of lone
  pairs around the central atom
• Single, Double and Triple bonds count as ONE
• 3. Use CHART to determine shape
• Name of molecule shape is based on position of
  atoms

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Warm-Up

• Welcome Back
• Remember to write the date, copy the question,
  then answer the question
• Draw the Lewis Structure for HPS
• What is the central atom?
• How many atoms are attached to the central atom
• How many lone pairs are on the central atom?

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Polyatomic Ion Quiz

• Bromate
• Acetate
• Acetate
• Chlorate
• Cyanide
• Chlorite
• Ammonium
• Perchlorate

2 different ways
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Linear
180
Trigonal Planar
120
Tetrahedral
109.5
Trigonal Pyramidal
lt109.5
Bent
lt120
Bent
lt109.5
14
Linear
Trigonal Planar
Tetrahedral
Trigonal Pyramidal
Bent
Bent
15
Linear
180
Trigonal Planar
120
Tetrahedral
109.5
Trigonal Pyramidal
lt109.5
Bent
lt120
Bent
lt109.5