Shapes of Molecules

In 1940 Sidgwick and Powell pointed out that the shape of molecules could be explained in terms of electron pair repulsions.

Electron pairs whether bonding or non-bonding repel each other and will arrange themselves in space to be as far apart as possible. Hence, the shape of a molecule is related to the number of outer electron pairs.

Here are the different structures:

 
  • No. of outer electrons pairs: 2
  • Bond angle: 1800
  • Example: BeCl2
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  • No. of outer electrons pairs: 3
  • Bond angle: 1200
  • Example: BF3, BCl3
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  • No. of outer electrons pairs: 4
  • Bond angle: 1090 and 270
  • Example: CH4
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  • No. of outer electrons pairs: 5
  • Bond angle: 1200 and 900
  • Example: PF5, PCl5
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  • No. of outer electrons pairs: 6
  • Bond angle: 900
  • Example: SF6

Valence shell electron pair repulsion theory:

  1. Valence shell electron pairs are arranged to minimise repulsions between themselves.
  2. Order of Repulsion Strength: Lone pair - lone pair → lone pair - bonding pair → bonding pair - bonding pair
  3. When there are 5 or more electrons pairs neglect repulsion between electron pairs at an angle greater than 900
  4. Basic geometry is still determined by number of electron pairs.
  5. Lone pair repulsion → Double bond repulsion → Bonding pair repulsion

Example: CH4

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  1. How many electrons around the central atom C = 4
  2. How many valence electrons shared by H = 4
  3. Number of bonding pairs = 4 + 4 / 2 = 4
  4. Number of lone pairs = 0
  5. Total number of electron pairs = 4
  6. Arrangement = Tetrahedral

The same calculation can be used for all molecules with single bonds.

Ammonia

This molecule has three bonding pairs of electrons and one lone pair. Although, based on the tetrahedral shape, due to the extra repulsion from the presence of a lone pair, the 3 N-H bonds bend further away from the lone pair, in order to minimise the repulsion.

The shape is described as pyramidal and has a bond angle of 1070.

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Water

This molecule consists of two lone pairs and two bonding pairs. Again, these repel each other towards the corners of the tetrahedron, due to the extra repulsion from the presence of two lone pairs. This shape is described as bent and has a bond angle of 1040.

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