Molecular Geometry

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Chapter 6.5

• Molecular Geometry

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Ch 6.5 Targets

• 6.5-1 I can predict the geometry of a molecule as
  either linear, trigonal planar, bent,
  tetrahedral, or trigonal pyramidal.
• 6.5-2 I can identify if a molecule would be
  likely to exhibit London dispersion forces,
  dipole-dipole forces, or hydrogen forces.

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How does DNA get its shape?
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Target

• 6.5-1 I can predict the geometry of a molecule as
  either linear, trigonal planar, bent,
  tetrahedral, or trigonal pyramidal.

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VSEPR Theory

• Valence Shell Electron Pair Repulsion Theory
• Valence electrons on atoms repel each other
• Atoms spread apart as far as possible from each
  other

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Steps to Determine Geometry

• Draw the Lewis Dot Structure for the molecule
• Identify how many groups of electrons surround
  the central atom.
• Identify the basic shape as either
• 2 groups linear
• 3 groups trigonal planar
• 4 groups tetrahedral

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Basic Geometries
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Steps to Determine Geometry

• 4. If there are any lone pairs of electrons
  determine whether the geometry is bent, or
  pyramidal.

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Shapes with Lone Pairs of Electrons
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Target Check

• 6.5-1 I can predict the geometry of a molecule
  as either linear, trigonal planar, bent,
  tetrahedral, or trigonal pyramidal.
• Explain what VSEPR theory says about the
  locations of atom around the central atom.

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Target Check

• 6.5-1 I can predict the geometry of a molecule
  as either linear, trigonal planar, bent,
  tetrahedral, or trigonal pyramidal.
• Draw a Lewis dot structure and predict what
  geometric molecular shape each molecule would
  have.
• NH3
• BCl3
• BeF2

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Target Check

• 6.5-1 I can predict the geometry of a molecule
  as either linear, trigonal planar, bent,
  tetrahedral, or trigonal pyramidal.
• Draw a Lewis dot structure and predict what
  geometric molecular shape each molecule would
  have.
• HI
• CBr4
• H2O

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Target

• 6.5-2 I can identify if a molecule would be
  likely to exhibit London dispersion forces,
  dipole-dipole forces, or hydrogen forces.

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Intermolecular Forces

• Weak forces of attraction between neutral
  molecules

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Intermolecular forces vs. Intramolecular forces
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London Dispersion Forces

• All molecules exhibit these forces
• All electrons move around the atom
• At any split second most electrons may be on one
  side of the atom
• The side of the atom with most electrons is
  slightly negative
• The side of the atom with less electrons is
  slightly positive
• Electrons in adjacent atoms synchronize their
  rotation causing attraction

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London Dispersion Forces

• Example Neon atoms have London dispersion forces

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Dipole-Dipole Forces

• Molecules with polar covalent bonds can have
  dipole-dipole forces in addition to London
  Dispersion forces.
• Unequal sharing of electrons causes there to be a
  slightly positive pole and a slightly negative
  pole.
• The slightly positive pole is attracted to the
  slightly negative pole of an adjacent molecule.

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Dipole-Dipole Forces

• Example HCl

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Hydrogen Bonding

• Occurs when hydrogen bonded to an atom of high
  electronegativity is attracted to a lone pair of
  electrons on another molecule.
• Ex. Water hydrogen bonds to itself.

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Target Check

• 6.5-2 I can identify if a molecule would be
  likely to exhibit London dispersion forces,
  dipole-dipole forces, or hydrogen forces.
• How are intermolecular forces different than
  bonds?

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Target Check

• 6.5-2 I can identify if a molecule would be
  likely to exhibit London dispersion forces,
  dipole-dipole forces, or hydrogen forces.
• What types of intermolecular forces would you
  expect to exist between two molecules of O2?

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Target Check

• 6.5-2 I can identify if a molecule would be
  likely to exhibit London dispersion forces,
  dipole-dipole forces, or hydrogen forces.
• What types of intermolecular forces would you
  expect to exist between two molecules of methanol
  (CH3OH)?
• Draw the Lewis Dot Structure for two methanol
  molecules and use a dotted line to show the
  location of the intermolecular force.

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Target Check

• 6.5-2 I can identify if a molecule would be
  likely to exhibit London dispersion forces,
  dipole-dipole forces, or hydrogen forces.
• What types of intermolecular forces would you
  expect to exist between two molecules of
  trichloromethane (formula CHCl3)?
• Draw a Lewis Dot Structure for two
  trichloromethane molecules. Use a dotted line to
  show the intermolecular force of attraction.