che00001che00005che00022 lecture 10

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VSEPR Theory Dr Graeme Jones
LECTURE 10
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Contents of lecture

• What is VSEPR theory?
• What are the rules?
• Structures and number of electron pairs
• Examples

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What is VSEPR theory?

• It stands for Valence Shell Electron Pair
  Repulsion theory.
• It is a way of predicting the shapes of molecules
  based on the number of pairs of valence (outer
  shell) electrons around the central atom in a
  molecule.
• It works well for about 95 of main group
  compounds.

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The rules of VSEPR theory

• Electron pairs are arranged around an atom so as
  to minimise their mutual repulsion.
• The order of repulsion for electron pairs is
  lone lone gt lone bond gt bond bond.
• The closer an electron pair is to the atom
  centre, the larger its repulsive effect on other
  electrons associated with the centre.

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Structures and the number of electron pairs

• The structure adopted by a molecule is related to
  the number of pairs of electrons around the
  central atom.
• The next slide gives the geometries adopted for
  up to 6 pairs of electrons.
• Elements like P and S can have more than 4
  pairs of electrons around them (the expanded
  octet effect)

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VSEPR geometries
http//en.wikipedia.org/wiki/VSEPR
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Examples

• BeH2
• BF3
• CH4
• PF5
• SF6

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Variations on tetrahedral geometry

• In CH4 all the bond angles are 109.47?.
• Some structures are predicted to have tetrahedral
  geometry but this is affected by the presence of
  lone pairs of electrons.
• We will look at NH3 and H2O/H2S to see the effect
  of lone pairs.

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NH3 1 lone pair

• We know from the Lewis structure that NH3 has 1
  lone pair of electrons.
• There are 4 pairs of electrons in total around
  the N, so we would expect a tetrahedral shape
  with the lone pair in one of the H positions
• The lone pair repels the bond pairs and reduces
  the bond angle to 107.5?

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NH3 actual structure

• The actual structure of NH3 is like a pyramid
• A similar structure is seen for the H3O ion
  (check the Lewis structure from lecture 9), where
  there is a single lone pair on the O.

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H2O and H2S

• We saw the Lewis structure of H2O in lecture 9.
  There are 4 pairs of electrons around O, and 2
  are lone pairs. The expected structure is
  tetrahedral.
• The effect of the two lone pairs is to compress
  the bond angle down to about 105?.
• H2S is similar, with 4 pairs of electrons, 2 of
  which are lone pairs.

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H2S structure

• The diagrams below show the structure of H2S with
  and without the lone pairs being shown

The bond angle is 92?, partly due to further lone
pair repulsion.
See http//www.newton.dep.anl.gov/askasci/chem03
/chem03447.htm
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Examples with more than one possible structure

• In more complex molecules there may be more than
  one possible structure, and VSEPR can be used to
  decide which one will be adopted, based on
  minimum repulsion between electron pairs.
• Remember, the order of repulsion for electron
  pairs is lone lone gt lone bond gt bond bond.

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SF4 2 possible structures

• S provides 6 electrons, and the F atoms each
  provide 1 electron, so there are 10 electrons or
  5 pairs.
• The predicted structure is trigonal bipyramidal,
  but there are 5 possible positions for 4 atoms.
• The 2 possible structures will be shown, and most
  stable predicted by VSEPR.

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SF4 structure
http//cnx.org/content/m12594/latest/
Why the deviation from 180? for the vertical bond
angle?
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ClF3 3 possible structures

• Cl provides 7 electrons, and the F atoms each
  provide 1 electron, so there are 10 electrons or
  5 pairs.
• The predicted structure is again trigonal
  bipyramidal, but now there are 5 possible
  positions for 3 atoms.
• The 3 possible structures will be shown, and most
  stable predicted by VSEPR.

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ClF3 structure

• The structure adopted by ClF3 is given below

http//www.chm.bris.ac.uk/motm/clf3/clf3.htm
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XeF2 and XeF4

• Xe can react with F2 to give 2 possible
  compounds, XeF2 and XeF4.
• Xe has 8 electrons in its outer shell
• In XeF2 there will be 10 electrons around Xe (5
  pairs), suggesting trigonal bipyramidal, but
  where do the 2 F atoms go?
• In XeF4, there will be 12 electrons around Xe (6
  pairs), suggesting octahedral, but where do the 4
  F atoms go?

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Structures adopted by XeF2 and XeF4
i.e. a linear structure
XeF2
XeF4
i.e. a planar structure
In all cases the structures adopted minimise
repulsion between lone pairs
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Examples

• Using VSEPR predict the structure of the
  following molecules.
• SF2 AlCl3 BrF3
• SiCl4 HOF CHCl3
• CF2Cl2 PH3 BrF5