Valence Shell Electron Pair Repulsion (VSEPR) Theory

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Valence Shell Electron Pair Repulsion (VSEPR)
Theory

• Chemistry 11
• November 7, 2008

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Vsepr Theory

• What is it?
• Model developed to predict the shapes of
  molecules
• Remember
• Atoms are bound together by electron pairs called
  bonding pairs
• These can be single (one pair e- single bond)
  or multiple (2 pair e- double bond 3 pair e-
  triple bond)
• Some atoms in a molecule can also have pairs of
  electrons not involved in bonding called lone
  pairs or non-bonded pairs

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VSEPR Theory

• Why are we concerned with the shape of molecules?
  
• Will help us later when we discuss polarity
• What causes molecules to take on a different
  shape?
• Electron pairs (bonding pairs and lone pairs) are
  negatively charged and will repel each other
• These pairs of electrons tend to occupy positions
  around the atom that minimize repulsions and
  maximize the distance of separation between them

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• Three types of repulsions take place in an atom
• Lone Pair Lone Pair (LP-LP)
• Lone Pair Bonding Pair (LP-BP)
• Bonding Pair Bonding Pair (BP-BP)
• Lone pairs occupy more space than bonding
  electron pairs
• Double bonds occupy more space than a single bond

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• A molecule must avoid these repulsions to remain
  stable.
• When repulsion cannot be avoided, the weaker
  repulsion (i.e. the one that causes the smallest
  deviation from the ideal shape) is preferred.
• Lone pair-lone pair (LP-LP) repulsion is
  considered to be stronger than the lone
  pair-bonding pair (LP-BP) repulsion, which in
  turn is stronger than the bonding pair-bonding
  pair (BP-BP) repulsion.
• LP-LP gt LP-BP gt BP-BP

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Things to Remember

• When assigning a VSEPR Shape to a molecule, we
  focus on the central atom and the bonding pairs
  or lone pairs associated with it
• Ex. CO2

• Electron pairs are considered to exist in a
  domain

• Domains can be made up of
• A lone pair
• A single bond
• A double bond
• A triple bond
• Thus, all are considered to be one electron pair

Domain
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Steps to Determine VSEPR Shape

1. Draw the Lewis structure of the molecule
2. Determine the central atom (the least
   electronegative)
3. Determine the number of bonding pairs
4. Determine the number of lone pairs
5. Consult the VSEPR chart to find the shape

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VSEPR Chart
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Number of Electron Groups Name of Molecular Shape Type of Electron Pairs Shape Example
2 Linear All Bonding Pairs CO2
3 Trigonal Planar All Bonding Pairs CH2O
4 Tetrahedral All Bonding Pairs CH4
4 Pyramidal 3 Bonding Pairs 1 Lone Pair NH3
4 Bent 2 Bonding Pairs 2 Lone Pairs H20
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• Carbon Dioxide CO2
• As a Lewis Dot diagram
• Has two electron pairs - two sets of double
  bonds two bonding pairs
• Creates a LINEAR shape according to VESPR

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• Methanal CH2O
• As a Lewis Dot diagram
• Has three electron pairs - three bonding pairs
  and no lone pairs
• Note Only has three bonding pairs because the
  double bond is considered to be one bonding pair!
• Creates a TRIGONAL PLANAR shape according to VESPR

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• Methane CH4
• As a Lewis Dot diagram
• Has four electron pairs - four bonding pairs and
  no lone pairs
• Creates a TETRAHEDRAL shape according to VESPR

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• Ammonia NH3
• As a Lewis Dot diagram
• Has four electron pairs - three bonding pairs
  and one lone pair
• Creates a PYRAMIDAL shape according to VESPR

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• Water H2O
• As a Lewis Dot diagram
• Has four electron pairs - two bonding pairs and
  two lone pairs
• Creates a BENT shape according to VESPR

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Practice

• Draw the VSEPR shape for each of these molecules

• HCN
• CCl4
• SI2
• H3O
• CH2F2
• AsCl3
• NO2
• CH3F

• NF3
• CCl2F2
• NCl3
• SO42-
• COCl2
• NO2-
• NH4

Finished? Assign VSEPR shapes to the homework
sheet (questions 1-5) from last class. ?