Recap Resonance
• Elements from the third period and beyond form
compounds with 8 or more electrons around the atom.
• This flexibility may result in quite different resonance
structure being possible.
• The resonance structure(s) with the greatest
contribution to the actual structure can be identified
using the valency of oxygen as a guide.
Example: ClO4-
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VSEPR Theory
Lewis structures give bonding arrangements but do not
imply any molecular shape. For this we use:
Valence Shell Electron Pair Repulsion Theory
This relies on minimising repulsion between areas of
electrons (bond pairs and lone pairs) around the central
atom.
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VSEPR Theory
1. Draw Lewis Structure.
2. Count number of electron pairs.
•
•
Count both bonding pairs and non-bonding pairs.
Count multiple bonds as only one area of electrons.
3. Determine the arrangement of electron pairs.
•
Electron pairs want to be as far away from each
other as possible.
4. Use atom positions to name molecular
geometry.
•
This is the atom positions.
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Electron Pair Arrangements
• Two electron pairs:
– Atoms at the opposite ends of a line.
– 180 degrees between areas of electrons.
– Called linear.
– eg CO2
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Electron Pair Arrangements
• Three electron pairs:
– Atoms at the corners of a triangle.
– 120 degrees between electron pairs.
– Called trigonal planar.
– Eg BF3
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Electron Pair Arrangements
• Four electron pairs:
– Atoms at the corners of a tetrahedron.
– 109.5 degrees between electron pairs.
– Called tetrahedral.
– Eg CH4
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Molecular Geometry
Remove one arm from the electron pair
arrangement for each lone pair present.
Trigonal Planar (3 e- pairs)
Figure 10.4 Silberberg
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Molecular Geometry
Tetrahedral (4 e- pairs)
Figure 10.5 Silberberg
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Molecular Geometry
• Repulsion: lone pair-lone pair > lone pairbond pair > bond pair-bond pair.
109.5
107
104.5
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Figure 10.9 Silberberg
Molecular Geometry
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Molecular Geometry - Example
• Molecules with multiple bonds eg COCl2 total 24 e-
~120
3 areas of electrons about C,
so trigonal planar
arrangement of electrons
No lone pairs so
molecular geometry is
also trigonal planar 11
Molecular Geometry - Example
• Cases when there is no single central atom
– Just apply the VSEPR rules to each central atom in
turn.
~120
3 areas of electrons about
each C, so trigonal planar
arrangement of electrons
about each C
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Dipole Moments
• Any bond between two different atoms will be
polar.
• A molecule has a permanent dipole moment if
it contains polar bonds and it is not a
symmetrical shape.
• Note: Cations and anions are not polar – the
overall charge overwhelms any local d+ vs deffects.
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Dipole Moments
• Polar molecules
HF
H2O
• Non-polar molecules
CHCl3
CCl4
N2
CO2
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Learning Outcomes:
• By the end of this lecture, you should:
− work out the number of bonding and non-bonding
pairs from the Lewis structure of a molecule
− predict the distribution of these pairs around an
atom
− predict and describe the molecular shape
− determine if a permanent dipole exists
− be able to complete the worksheet (if you haven’t
already done so…)
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Questions to complete for next lecture:
1. Draw the shapes of the following molecules and ions and
give approximate bond angles
(a) BH3
(b) NH4+
(c) CS2
(d) CH2O
(e) CH3Cl
(f) H3O+
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Questions to complete for next lecture:
2. What are the approximate C-C-C bond angles in the
two molecules below?
3. Are these molecules flat?
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