Transcript VSEPR and shapes of molecules

Shapes of Molecules
David Read
Key Aims
• Revise A-level VSEPR theory.
• Prepare for Phil Gale’s lectures
on symmetry, isomers, NMR etc.
Warm-up:
How many different molecular
shapes can you think of?
Give an example for each shape
and sketch the structure.
How many shapes did you identify?
4
5
6
7
8
9
10
>10
25%
20%
20%
18%
7%
7%
2%
>1
0
10
9
8
7
6
5
0%
4
A.
B.
C.
D.
E.
F.
G.
H.
Underlying principle
(see Shriver & Atkins)
• In the VSEPR model, electron
regions
of enhanced
pairs
take up electron
positionsdensity
as far
take up
apart
aspositions
possible.as far apart as
possible.
• EPs can be bonding pairs
(BPs) or lone pairs (LPs).
How confident do you feel about
using VSEPR to determine shapes?
58%
1. It’s a doddle!
2. OK, but need to revise it a bit.
27%
3. Not too confident.
I don’t remember this
at all!
I don’t remember
much.
Not too confident.
OK, but need to
revise it a bit.
5. I don’t remember this at all!
4%
2%
It’s a doddle!
4. I don’t remember much.
9%
Step-by-step
1. Write a Lewis dot structure for the molecule.
* A Lewis dot structure is like a dot-and-cross
* diagram that only uses dots.
e.g. methane
C – 4 valence electrons
H – 1 valence electron
All 4 valence electrons on
C pair up with an electron
from an H atom, forming 4
bonds.
Step-by-step
2. Count the number of atoms and lone pairs
around the central atom.
e.g.
4 atoms around carbon,
and no lone pairs.
4 regions of electron
density to distribute.
Step-by-step
3. Work out how the regions of high electron
density will be arranged around the central
atom:
2 Regions – Linear
3 Regions – Trigonal planar
4 Regions – Tetrahedral
5 Regions – Trigonal bipyramidal
6 Regions – Octahedral
Step-by-step
4. Draw the structure and label bond angles.
109.5º
Tetrahedral
Step-by-step
4. Draw the structure and label bond angles.
109.5º
Tetrahedral
Try one yourself…
•
•
•
•
What is the shape of a PF5 molecule?
Lewis structure 
Count e- pairs around central atom  5
How many lone pairs are there?  0
Draw structure and label bond angles.
Press any button on your
handset when you have
completed the task.
PF5
The 5 bonding pairs will take up a trigonal
bipyramidal arrangement:
PF5
The 5 bonding pairs will take up a trigonal
bipyramidal arrangement:
What is the shape of BeH2 ?
1. Tetrahedral
2. Trigonal pyramidal
3%3%
14%
6%
3. Linear
4. V-shaped
5. Octahedral
75%
Tetrahedral
Trigonal pyramidal
Linear
V-shaped
Octahedral
Beryllium hydride
• Be – 2 valence electrons
• 2 bonds formed to H atoms
• Linear molecule
180º
What is the shape of BF3 ?
1. Tetrahedral
2. Trigonal planar
0%6%
3. Linear
94%
4. V-shaped
5. Octahedral
Tetrahedral
Trigonal planar
V-shaped
Octahedral
Linear
Boron trifluoride
• B – 3 valence electrons
• 3 bonds formed to F atoms
• Trigonal planar molecule
120º
What is the shape of SF6 ?
1. Tetrahedral
2. Trigonal pyramidal
0%
3%
0%
3. Linear
97%
4. V-shaped
5. Octahedral
Tetrahedral
Trigonal pyramidal
Linear
V-shaped
Octahedral
Sulphur hexafluoride
• S – 6 valence electrons
• 6 bonds formed to F atoms
• Octahedral molecule
90º
What about lone pairs?
Which statement is correct?
1. Repulsion due to bonding
pairs is greater than
repulsion due to lone pairs.
2. Repulsion due to bonding
pairs is the same as
repulsion due to lone pairs.
3. Repulsion due to lone pairs
is greater than repulsion
due to bonding pairs.
14%
5%
81%
A
B
C
Step-by-step
1. Write a Lewis structure for the molecule.
e.g. water
O – 6 valence electrons
H – 1 valence electron
2 of oxygen’s 6 valence
electrons pair up with an
electron from an H atom,
forming 2 bonds. The
remaining valence
electrons make up 2 LPs.
Step-by-step
2. Count the number of bonds and lone pairs
around the central atom.
e.g. Water
2 bonds and 2 LPs
4 regions of
electron density
to distribute
Step-by-step
3. Work out how the electron regions will be
arranged around the central atom:
2 Regions –
3 Regions –
4 Regions –
5 Regions –
6 Regions –
Linear
Trigonal planar
Tetrahedral
Trigonal bipyramidal
Octahedral
Step-by-step
4. Deduce the shape of the molecule based on
the arrangement of atoms (not including LPs)
Step-by-step
5. Draw the structure.
What about the bond angle?
Methane, ammonia and water
4 bonds, 0 lone pairs
3 bonds, 1 lone pair
2 bonds, 2 lone pairs
Tetrahedral
Trigonal pyramidal
Bent (v-shaped)
Bond angle = 109.5º
This is the ideal tetrahedral angle
What’s the bond angle in water?
A. 109.5º
B. 108º
7% 7%
C. 104.5º
86%
109.5º
108º
104.5º
What’s the bond angle in
ammonia?
A. 109.5º
B. 108º
5%
8%
C. 104.5º
88%
109.5º
108º
104.5º
Methane, ammonia and water
4 bonds, 0 lone pairs
3 bonds, 1 lone pair
2 bonds, 2 lone pairs
Tetrahedral
Trigonal pyramidal
Bent (v-shaped)
Bond angle = 109.5º
Bond angle = 108º
Bond angle = 104.5º
One lone pair reduces the bond angle slightly
Two lone pairs reduce the bond angle more
Try another one yourself…
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•
•
•
What is the shape of a XeF4 molecule?
Lewis structure 
Count e- regions around central atom  6
How many lone pairs are there?  2
Draw structure and label bond angles.
Press any button on your
handset when you have
completed the task.
Xenon tetrafluoride
• Xe – 8 valence electrons
• 4 bonds formed to F atoms
• 4 electrons left = 2 LPs
• 6 regions  octahedral
• 4 regions are bonds 
Square planar molecule
Xenon tetrafluoride
• Xe – 8 valence electrons
• 4 bonds formed to F atoms
• 4 electrons left = 2 LPs
• 6 regions  octahedral
• 4 regions are bonds 
Square planar molecule
Bond angle = 90º
What about double bonds?
Carbon dioxide
• C – 4 valence electrons
• 2 double bonds formed to O atoms
(all valence electrons used in bonds)
• 0 lone pairs
• 2 regions (both are bonds) 
Linear molecule
Carbon dioxide
• C – 4 valence electrons
• 2 double bonds formed to O atoms
(all valence electrons used in bonds)
• 0 lone pairs
• 2 regions (both are bonds) 
Linear molecule
Try another one yourself…
•
•
•
•
What is the shape of a SO2 molecule?
Lewis structure 
Count e- regions around central atom  3
How many lone pairs are there?  1
Draw structure and label bond angles.
Press any button on your
handset when you have
completed the task.
What is the shape of SO2 ?
1. Tetrahedral
2. Trigonal pyramidal
3. Linear
4. V-shaped
5. Octahedral
Sulphur dioxide
• S – 6 valence electrons
• 2 double bonds formed to O atoms
(4 valence electrons used in bonds)
• 2 electrons left  1 lone pair
• 3 regions  trigonal planar
• 2 bonds to atoms  v-shaped
Linear molecule
Challenge…
What are the shapes of IF7 and IF5 molecules?
Challenge…
What are the shapes of IF7 and IF5 molecules?
Pentagonal bipyramidal
Challenge…
What are the shapes of IF7 and IF5 molecules?
Pentagonal bipyramidal
Square pyramidal
How confident do you feel about
using VSEPR to determine shapes?
1. It’s a doddle!
64%
2. OK, but need to revise it a bit.
4. I don’t remember much.
2%
2%
I don’t remember
much.
I don’t remember this
at all!
OK, but need to
revise it a bit.
It’s a doddle!
5. I don’t remember this at all!
5%
Not too confident.
3. Not too confident.
27%
How useful did you find the zapper
session?
1. Very Useful
2. Quite useful
2%
0%
0%
3. No view either way
44%
53%
4. Not very useful
5. Waste of time
Very Useful
Quite useful
No view either way
Not very useful
Waste of time