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Determining Molecular Shape
Valence Shell Electron Pair Repulsion Theory (VSEPR)
 
In order to determine the geometry of a molecule, we must consider the number and type of
valence electrons around given atoms within that molecule
...
Those that are involved in bonding are known as
bonding pairs, and those that aren’t are known as lone pairs
• Electron pairs repel each other
...
This means that electron pair
repulsion increases in the series:
bonding pair – bonding pair < bonding pair – lone pair < lone pair – lone pair
To work out the geometry around a given atom use the following steps:
1
...
Decide which atom you want to look at
3
...
Use the number of electron pairs, bonding pairs and lone pairs to determine the
geometry

Counting the Electrons
 

1
...
Add electrons for various substituents
- Single bonds, add one electron
- Double bonds add two electrons
- Triple bonds, add three electrons
3
...
Divide the number of total electrons by two to give the number of valence electron pairs
(VEP)
5
...


If there are multiple bonds present around the atom in question, then simply deduct one pair
of electrons for each π bond, as this gives the number of σ electron pairs present
...

VEP determines electron pair geometry in the following way:
VEP
2
3
4
5
6

Electron Pair Geometry
Linear
Trigonal planar
Tetrahedral
Trigonal bipyramidal
Octahedral

The table above shows how VEP relates to electron geometry when lone pairs are not
involved, but as mentioned in the assumptions of VSEPR lone pairs take up more space than
bonding pairs
...

VEP

2

3

4

5

6

0 lp

1 lp

2 lp

3 lp

4 lp

VSEPR Examples
 

In order to determine
molecular geometry simply
follow the steps detailed
earlier
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The Effect of Lone Pairs
As mentioned earlier, lone pairs occupy more space than bonding pairs of electrons
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For example:
H
N

H

H

H

H

H

H

106
...
5°
O
H

The lone pair of electrons on the nitrogen repels the bonding
pairs of electrons between the nitrogen and hydrogen atoms
...

The two lone pairs in water mean a greater repulsion of bonding
pairs again, and so the bond angles are reduced even further
...
5°

Effect of Electronegative Groups
The presents of substituents with different electronegativities can also distort bond angles as
the pull felt by the electrons towards the different atoms has changed
...
This means that there is more space around the central
H
carbon atom for the carbon-hydrogen bonds to spread out
...
5°
principle applies to other examples of electronegative groups being
H
present
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5°

Effect of Multiple Bonds
Multiple bonds have more electrons present in them and so they take up more space than
single bonds
...

For example:
O

114°

P
Cl

104°

Cl
Cl

In this example, POCl3 has a distorted tetrahedral shape
...
The outcome is that the O-P-Cl bond
angle is larger than the Cl-P-Cl bond angle
...
There
are two axial positions and three equatorial positions
...

a

A good way to demonstrate this is using a molecule modelling kit,
although some matchsticks and blu-tack work just as well
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Another example of a molecule with this shape is sulphur tetrafluoride, SF4
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The lone pair on sulphur takes up one of the equatorial
positions as this decreases the repulsions from neighbouring pairs of electrons
...
For this reason, the structures where the lone
pairs are in the equatorial positions are favoured reducing the 90° repulsions as much as
possible
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Compounds with Multiple Centres
So far we’ve only looked at compounds that have a single central atom to which all other atoms
are bonded
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The same rules and steps apply
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