1 Recap VSEPR Theory 1. Draw Lewis Structure. 2. Count number of electron pairs. 3. Determine the arrangement of electron pairs. 4. Use atom positions to name molecular geometry.
Feb 23, 2016
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Recap VSEPR Theory1. Draw Lewis Structure.2. Count number of electron pairs.3. Determine the arrangement of electron
pairs.4. Use atom positions to name molecular
geometry.
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Three areas of electrons: Trigonal Planar• Remove one arm from the electron pair
arrangement for each lone pair present.
Recap
Figure 10.4 Silberberg
SO3 SO2
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Four areas of electrons: Tetrahedral
Recap
Figure 10.5 Silberberg
CH4 NH3 H2O
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Electron Pair Arrangements
• Five electron pairs:– Atoms at the corners of a trigonal
bipyramid.– Some electron pairs separated by 120
degrees, other by 90 degrees.– Called trigonal bipyramidal.
Figure 10.2 Silberberg
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Electron Pair Arrangements
• Six electron pairs:– Atoms at the corners of an octahedron.– 90 degrees between electron pairs.– Called octahedral.
Figure 10.2 Silberberg
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Where do lone pairs go?• Five electron pairs:
– If one lone pair present is it in an axial or equatorial position?
– Repulsion: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.
Equitorial
Axial
Structures derived from a trigonal bipyramid
Molecular Geometry
Figure 10.7 Silberberg
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Structures derived from an octahedron
Molecular Geometry
Figure 10.8 Silberberg
All positions are identical
Number of lone electron pairs0 1 2 3
3
trigonal planar bent 4
tetrahedraltrigonal
pyramidal bent 5
trigonal bipyramidal see-saw T-shaped linear
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octahedralsquare
pyramidal square planar T-shaped
Tota
l num
ber o
f ele
ctro
n pa
irsSummary of Molecular Geometry
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Molecular Geometry and Polarity
BF3 ClF3F Cl
F
F
PolarNon-Polar
F B
F
F
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Molecular Geometry and Polarity
SiF4 SF4
PolarNon-Polar
F
SiF F
F
F
SF F
F
• By the end of this lecture, you should:− be able predict the molecular geometry of
molecules and ions containing a central atom
− be able to name the geometry of a given species
− understand the positioning of lone pairs to minimise repulsion
− be able to predict the presence of a permanent dipole moment
− be able to complete the worksheet (if you haven’t already done so…)
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Learning Outcomes:
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Questions to complete for next lecture:
1. Give the geometry of the following polyatomic ions and give approximate bond angles.a) AsF6
-
b) PBr4-
c) SF5-
d) SO42-
e) ICl2-
f) N3-
2. A binary compound contains two different elements. What molecular geometries would result in binary compounds that are polar?
3. What molecular geometries would result in binary compounds that are non-polar?