Recap – Last Lecture
VSEPR Theory
•  Lewis structures give us a stable arrangement of bonds and lone
pairs.
Lewis structures give bonding arrangements but do not
imply any molecular shape. For this we use:
•  Based on 8 e- for C, N, O & F.
•  8, 10 or 12 e- for P, S, Cl, Br, I (when these are the central atoms).
•  Resonance occurs where two or more valid Lewis structures may be
drawn which differ only in position of electrons.
•  The actual structure is a weighted average of resonance structures
and is more stable than expected.
Valence Shell Electron Pair Repulsion Theory
This relies on minimising repulsion between areas of
electrons (bond pairs and lone pairs) around the central
atom.
•  The resonance structure(s) with the greatest contribution to the
actual structure can be identified using the valency of oxygen as a
guide.
1
VSEPR Theory
2
Electron Pair Arrangements
1.  Draw Lewis Structure.
•  Two electron areas:
2.  Count number of electron areas.
•  Count both bonding pairs and non-bonding pairs.
•  Count a multiple bonds as one area of electrons.
3.  Determine the arrangement of electron areas.
–  Atoms at the opposite ends of a line.
–  180º between areas of electrons.
–  Called linear.
–  eg CO2
•  Electron pairs want to be as far away from each
other as possible.
O
4.  Use atom positions to name molecular
geometry.
C
O
3
Electron Pair Arrangements
4
Electron Pair Arrangements
•  Three electron areas:
•  Four electron areas:
–  Atoms at the corners of a triangle.
–  120º between electron pairs.
–  Called trigonal planar.
–  Eg BF3
–  Atoms at the corners of a tetrahedron.
–  109.5º between electron pairs.
–  Called tetrahedral.
–  Eg CH4
H
F
BF3
B
F
CO2
F
C
H
H
CH4
H
5
6
1
Electron Pair Arrangements
Electron Pair Arrangements
•  Five electron areas:
•  Six electron areas:
–  Atoms at the corners of a trigonal bipyramid.
–  Some electron pairs separated by 120º
degrees, other by 90º.
–  Called trigonal bipyramidal.
–  Atoms at the corners of an octahedron.
–  90º between electron pairs.
–  Called octahedral.
F
F
Cl
Cl
Cl
F
S
P
Cl
PCl5
F
SF6
F
F
Cl
7
Molecular Geometry
8
Molecular Geometry - Example
Remove one arm from the electron area
arrangement for each lone pair present.
•  Molecules with multiple bonds eg COCl2 total 24 e-
Trigonal Planar (3 electron areas)
~120°
Figure 10.4 Silberberg
9
Molecular Geometry
3 areas of electrons about C,
so trigonal planar
arrangement of electrons
No lone pairs on C so
molecular geometry is
also trigonal planar 10
Molecular Geometry
Tetrahedral (4 electron areas)
•  Repulsion: lone pair-lone pair > lone pairbond pair > bond pair-bond pair.
109.5°
11
107°
104.5°
12
2
Molecular Geometry
Molecular Geometry
Structures derived from a trigonal bipyramid
Structures derived from an octahedron
All positions are
identical
Summary of Molecular Geometry
Total number of electron areas
0 Number of lone pairs of electrons 1 2 3 3 trigonal planar bent tetrahedral trigonal pyramidal bent trigonal bipyramidal see-saw T-shaped linear octahedral square pyramidal square planar T-shaped 4 5 14
Applications – shape and function
•  Morphine is an alkaloid derived from the opium poppy. It is effective at blocking
the perception of pain by the brain while allowing the normal function of the
nervous system.
•  A derivative is heroin, which is devastatingly addictive. Heroin users become
physically dependant on opioids.
•  Methadone is used to treat heroin addiction. It is an agonist i.e. it binds to the
opioid receptors and causes an opioid response. It is prescribed as a way of
regulating and ultimately reducing heroin addiction.
•  Naloxone is an antagonist (i.e. it binds to the opioid receptors but does not give
a response) used to treat heroin overdose. It binds to the opioid receptors in
the brain, displacing heroin and reversing the effects of the narcotic (eg
respiratory depression).
6 Learning Outcomes:
16
Questions to complete for next lecture:
•  By the end of this lecture, you should be able to:
−  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.
−  place any lone pairs in appropriate positions to
minimize the overall electron pair repulsion.
−  predict and describe the molecular shape.
1.  Draw the shapes of the following molecules and ions.
Name the molecular geometry and give approximate
bond angles.
(a) SO3
(b) NH4+
(c)  PBr4(d) CH2O
(e) ICl2-
−  be able to complete the worksheet (if you haven’t
already done so…).
(f)  H3O+
17
18
3