Molecular Shape

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Molecular Shape
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Objectives

• 1. Define VSEPR theory
• 2. Predict the shapes of molecules using VSEPR
  theory

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Vocab

• VSEPR theory
• Bonding pair (shared pair)
• Unshared pair

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• What are the shortcomings of molecular and
  structural formulae?

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• Sucrose

• Sucralose

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Valence Shell Electron Pair Repulsion Theory

• VSEPR theory predicts some molecular shapes based
  on the idea that pairs of valence electrons
  surrounding an atom repel each other

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Basic Rules of VSEPR

• 1. Valence electrons surrounding the central
  atom determine shape
• 2. All valence electron pairs will try to be as
  far apart as possible
• 3. Most compounds conform to 3 basic shapes

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Shape 1, Linear

• When there are just 2 pairs of electrons
  surrounding the central atom, they form a linear
  shape
• They are the maximum distance from each other,
  forming a 180 angle
• Also, all diatomic molecules are linear
• Lets look at BeCl2

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Shape 2, Trigonal Planar

• When there are 3 pairs of electrons around the
  central atom, they form trigonal planar shapes
• The maximum angle formed is now 120
• Lets look at BF3

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Shape 3, Tetrahedral

• When there are 4 pairs of electrons surrounding
  the central atom, the tetrahedral shape is formed
• The maximum angle is now 109.5
• Lets look at CH4

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Unshared Pairs

• All of the examples so far have shown valence
  electrons as shared or bonded pairs
• What affect do you think unshared pairs have on
  molecules?
• Lets look at SnCl2

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• VSEPR states that unshared pairs will take up as
  much space as shared pairs
• This means that they will have the same geometric
  patterns, but different physical shape

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Shape 4, Bent

• The central atom has 2 bonded pairs and either 1
  or 2 unshared pairs
• The bent shape can come from either the
  tetrahedral or trigonal planar arrangements

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Shape 5, Trigonal Pyramidal

• The central atom has 3 bonded pairs and 1
  unshared pair
• This will have the same geometric pattern as the
  tetrahedral, so the bond angle will be 109.5