Valence Shell Electron Pair Repulsion VSEPR theory

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Valence Shell Electron Pair Repulsion (VSEPR)
theory

• Mrs. Kay

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Assumptions for VSEPR

• Atoms in a molecule are bound together by
  electron pairs. These are called bonding pairs.
  More than one set of bonding pairs of electrons
  may bind any two atoms together (multiple
  bonding).
• 1 pair single bond
• 2 pair double bond
• 3 pair triple bond

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• Some atoms in a molecule may also possess pairs
  of electrons not involved in bonding. These are
  called lone pairs or non-bonded pairs.

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• The bonding pairs and lone pairs around any
  particular atom in a molecule adopt positions in
  which their mutual interactions are minimized.
• Electron pairs are negatively charged and will
  get as far apart from each other as possible.

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Bonding angles

• Lone pairs occupy more space than bonding
  electron pairs.
• Double bonds occupy more space than single bonds.
  
• LP-LP gt LP-BP gt BP-BP
• Lone pairs spread out more than bonding pairs,
  therefore lone pairs have less repulsions than
  bonding pairs

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Where to start?

• Draw the Lewis structure based upon the atom of
  interest. Ex CO2
• Determine the central atom (pick the least
  electronegative atom) Ex C

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• Determine the number bonding pairs Ex 2
• Determine the number of lone pairs or non-bonding
  pairs. Ex 0
• sets bonding pairs (single, double, or triple)

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5. Using the chart figure out the shape.
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Put to memory!!

• The predicted geometry of the molecule is based
  on the number of

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Make your table

• As a group we will organize a table that will
  help you decide molecular shapes.

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Practice

• Use VSEPR to predict the following shape
• SO2
• SO3
• SO4 -2
• AsCl3
• SI2
• CH3F

• CH2F2
• NH4
• NO2-
• NO2
• H3O

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Advanced chemistry ONLY

• You are responsible for 2 extra shapes
• Triagonal bipyramidal 5 electron pairs
• Octahedral 6 electron pairs

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Trigonal Bipyramidal

• Molecules shaped by five electron pairs
• ExamplePCl5 phosphorous pentachloride

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• Elements in row 3,4,... have access to d orbitals
  (unlike row 2 elements that have only s and p
  orbitals) and can therefore have more than 8
  electrons around them in certain compounds. 
• Recall that P has a normal valence of 3 with one
  lone pair. 
• If that lone pair is divided into two extra
  unpaired electrons to be used in creating bonds,
  we then have a hypervalence of 5.  

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Octahedral

• Six bonding sets
• Examplehexafluorophosphide PF6-

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On the computer

• Exercise to practice on calculator
• http//www.shef.ac.uk/chemistry/vsepr/jmol/exercis
  es.html