VSEPR THEORY (Valence Shell Electron Pair Repulsion Theory)

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VSEPR THEORY (Valence Shell Electron Pair
Repulsion Theory)

• Adapted by Mr. M. McIsaac
• Carleton North High School, Bristol, NB
• From Mr. James Montgomery
• SCH4U Grade 12 Chemistry
• Sir John A. MacDonald Secondary School
• Waterloo, ON
• sjam.wrdsb.on.ca/files/Lesson14vsepr.ppt

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What Is The VSEPR Theory?

• VSEPR Theory is used to predict the shapes of
  molecules.
• Think of bonded pairs (shared) or lone pairs
  (nonbonded, unshared) of e-s as negatively
  charged clouds that repel each other.
• To achieve the most stable condition the clouds
  must be as far apart as possible in 3-D, thereby
  decreasing repulsion.
• The amount of repulsion can be ordered
• LP-LP gt LP-BP gt BP-BP
• In order to determine the shape, the Lewis
  diagram must be drawn first.

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2 Bond Pairs/Electron Groups

• Molecules that only have 2 bonding pairs on the
  central atom will have a LINEAR SHAPE with a bond
  angle of 180
• e.g. BeF2, CO2, CS2
• General Formula AX2
• Central atom A from group 2 2 BP 0 LP

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Example BeF2
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3 Bond Pairs/Electron Groups

• Molecules that have 3 bonding pairs on the
  central atom will have a TRIGONAL PLANAR SHAPE
  with bond angles of 120.
• e.g. BF3, BH3
• General Formula AX3
• Central atom A from group 13 3 BP 0 LP

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Example BF3
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4 Bonding Pairs/Electron Groups

• If the central atom is placed at the center of a
  sphere, than each of the four pairs of electrons
  will occupy a position to be as far apart as
  possible.
• This will result in the electron pairs being at
  the corners of a regular tetrahedron, therefore
  these molecules are said to have a TETRAHEDRAL
  SHAPE.
• The angle between each bond will be 109.5
• e.g. CCl4, CH4, SiH4
• General Formula AX4
• Central atom A from group 14 4 BP 0 LP

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Example CCl4
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3 Bonding Pairs 1 Non-bonding Pair

• Four pairs of electrons will always arrange
  themselves tetrahedrally around the central atom.
• The shape of the molecule is determined by the
  arrangement of the atoms not the electrons.
• As a result such molecules will have a TRIGONAL
  PYRAMIDAL shape.
• Due to the repulsion, a non-bonding electron pair
  requires more space than a bonding pair, the
  angles in these molecules are 107 not 109.5 as
  in the tetrahedral molecules.
• e.g. NH3, PCl3
• General Formula AX3E
• Central atom A from group 15 3 BP 1 LP

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Example NH3
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2 Bonding Pairs 2 Non-bonding Pairs

• The four pairs of electrons will be arranged
  tetrahedrally but since only 2 pairs are bonding
  electrons, the surrounding atoms are at 2 corners
  of the tetrahedron.
• As a result these molecules will have a V-SHAPE
  or BENT.
• The repulsion between the non-bonding pairs will
  result in a bond angle of 104.5.
• For each pair of non-bonding electrons, the bond
  angle decreases by 2.5
• e.g. H2O, H2S, OCl2
• General Formula AX2E2
• Central atom A from group 16 2 BP 2 LP

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Example H2O
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Things to Remember

• In order to predict the shape of a molecule you
  must draw the Lewis Dot Diagram for the molecule,
  determine the number of bonding and non-bonding
  electron pairs and compare this with the chart
  you have been given (the shapes must be
  memorised).
• When determining the shape of a molecule with
  multiple bonds, treat the multiple bonds as if
  they were single bonds (i.e. one bonding pair)

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5 Molecular ShapesFromSingle Bonds