Shapes of molecules
Shapes of molecules


The shapes of
molecules and ions
can be described by
valence shell electron
pair repulsion
VSEPR predicts the
shapes and bond
angles of simple
molecules
It’s all about the pairs


All of the electrons in an ion or molecule are in
organised into pairs
There are two types of electron pairs:

Bonding pairs


Lone pairs


(two electrons in a pair not involved in bonding – also known as
non-bonding pairs).
Electron pairs will repel each other as far as
possible.


(the two shared electrons in a covalent bond)
Like charges repel
This causes the shape of any molecule or ion
It’s all about the pairs

Chemists sometimes use lines to show bonds
XX
H
X
O
Cl
X
X
becomes H — Cl
XX

Draw out the following molecules just using a line to
show the bonds


O2 , H2O, CH4 , NH3 , SF6 , BF3 and PF5
What is the problem when
you draw these molecules
onto a piece of paper?
Beryllium Fluoride
Simple rules to work it out:
Draw a stick diagram.
1.
(displayed formula technically!)
Add in any lone pairs
onto the stick diagram.
Count the total number
of bonds AND lone pairs
to work out the shape.
2.
3.
OO
O
O
F
OO
OO
O
X
Be
X
O
F
LINEAR

O
O

2 Bonding pairs
0 Lone pairs
OO

The furthest these
pairs of electrons can
spread apart is 180o
Boron Trifluoride
Simple rules to work it out:
1.
Draw a stick diagram.
B
(displayed formula technically!)
2.
3.
Add in any lone pairs
onto the stick diagram.
Count the total number
of bonds AND lone pairs
to work out the shape.
F
F
XO
O
O
F
O
O
OO
TRIGONAL PLANAR



3 Bonding pairs
0 Lone pairs
The furthest these
pairs of electrons can
spread apart is 120o
1200 provides the greatest distance EVEN IN 3-dimensional space
Further examples
TETRAHEDRAL


4 Bonding pairs
0 Lone pairs
HEXAGONAL


6 Bonding pairs
0 Lone pairs
Double and triple bonds
This phosphate ion PO43- has a
tetrahedral shape
Double and triple bonds
do not effect the shape
(well not enough that we have to worry!)



TETRAHEDRAL
4 bonding pairs (even
though one is double)
0 Lone pairs
The furthest these
pairs of electrons can
spread apart is 109.5o
5 Bonding pairs

5 bonding pairs and 0 lone pairs is called:

TRIGONAL BI-PYRAMIDAL
It is a combination
of the 1st and 2nd
examples: linear
and trigonal planar
So what would the
TWO different bond
angles be?
Practice to fluency



Decide the shape and bond
angle(s) of the following
molecules on the right
NO3-
Prepare an explanation for
the rest of the class
explaining the shape and
bond
The fact that some of
these molecules are ions is
irrelevant to the shape!
CO2
PF5
SF6
CH4
SO42-
Lone pairs vs. bonding pairs


Lone pairs are more compact as they are closer to the
nucleus than bonding pairs
This causes lone pairs to provide more repulsion
LP-LP
LP-BP
BP-BP
•The greatest repulsion is between two lone pairs
•Followed by the repulsion between one lone pair
and one bonding pair
•The weakest repulsion is between two bonding pairs
Ammonia
PYRAMIDAL



Ammonia has 3 bonding pairs and 1 lone pair
With 4 pairs in total it is similar to tetrahedral
However the LP repels the bonding pairs and reduces the
bond angle by 2.5o from 109.5o to 107o
Water
NON-LINEAR (BENT)



Water has 2 bonding pairs and 2 lone pair
With 4 pairs in total it is similar to tetrahedral
However the TWO LP’s repels the bonding pairs and reduces
the bond angle by 2.5o and 2.5o from 109.5o to 104.5o
3D diagrams


To draw 3D diagrams on 2D
paper chemists draw the
bonds slightly differently
Solid wedges show a bond
that comes ‘up’ from the
paper or screen


(towards you)
Hashed wedges show a
bond that goes ‘down’ from
the paper

(away from you)
methane
TETRAHEDRAL
XY6
Bond angles?
“Real” shape?
Name?
Examples?
XY5
Bond angles?
“Real” shape?
Name?
Examples?
XY4
Bond angles?
“Real” shape?
Name?
Examples?
XY3
Bond angles?
“Real” shape?
Name?
Examples?
XY2
Bond angles?
“Real” shape?
Name?
Examples?
RECAP
Molecules, or ions, possessing ONLY BOND PAIRS of
electrons fit into a set of standard shapes. All the bond
pair-bond pair repulsions are equal.
All you need to do is to count up the number of bond
pairs and chose one of the following examples...
BOND
PAIRS
SHAPE
C
A covalent bond will repel
another covalent bond
BOND
ANGLE(S)
EXAMPLE
2
LINEAR
180º
BeCl2
3
TRIGONAL PLANAR
120º
AlCl3
4
TETRAHEDRAL
109.5º
CH4
5
TRIGONAL BIPYRAMIDAL
90º & 120º
PCl5
6
OCTAHEDRAL
90º
SF6
IRREGULAR SHAPES
If a molecule, or ion, has lone pairs on the central atom, the shapes are slightly
distorted away from the regular shapes. This is because of the extra repulsion
caused by the lone pairs.
BOND PAIR - BOND PAIR
O
<
LONE PAIR - BOND PAIR
O
<
LONE PAIR - LONE PAIR
O
As a result of the extra repulsion, bond angles tend to
be slightly less as the bonds are squeezed together.
Practice to fluency


Decide the shape and bond
angle(s) of the following
molecules on the right
BeCl2
AlCl3
For each example include:





Name
Bond angle(s)
3-D diagram
The number of bonding pairs
The number of lone pairs
CH4
NH4+
BERYLLIUM CHLORIDE
Be
Cl
Cl
Be
Cl
Beryllium - has two electrons to pair up
Two covalent bonds are formed
Chlorine - needs 1 electron for ‘octet’
Beryllium still has an incomplete shell
BERYLLIUM CHLORIDE
Be
Cl
Cl
Be
Cl
Beryllium - has two electrons to pair up
Two covalent bonds are formed
Chlorine - needs 1 electron for ‘octet’
Beryllium still has an incomplete shell
BOND PAIRS
2
LONE PAIRS
0
180°
BOND ANGLE...
SHAPE...
180°
LINEAR
Cl
Be
Cl
ALUMINIUM CHLORIDE
Al
Cl
Cl
Cl
Al
Aluminium - has three electrons to pair up
Cl
Chlorine - needs 1 electron to complete ‘octet’
Three covalent bonds are formed; aluminium
still has an incomplete outer shell.
BOND PAIRS
3
LONE PAIRS
0
Cl
120°
Cl
BOND ANGLE...
Al
120°
Cl
SHAPE...
TRIGONAL PLANAR
ALUMINIUM CHLORIDE
Al
Cl
Cl
Cl
Al
Aluminium - has three electrons to pair up
Cl
Chlorine - needs 1 electron to complete ‘octet’
Three covalent bonds are formed; aluminium
still has an incomplete outer shell.
BOND PAIRS
3
LONE PAIRS
0
Cl
120°
Cl
BOND ANGLE...
Al
120°
Cl
SHAPE...
TRIGONAL PLANAR
METHANE
H
C
H
H
C
H
H
Carbon - has four electrons to pair up
Four covalent bonds are formed
Hydrogen - 1 electron to complete shell
C and H now have complete shells
METHANE
H
H
H
C
C
H
H
Carbon - has four electrons to pair up
Four covalent bonds are formed
Hydrogen - 1 electron to complete shell
C and H now have complete shells
BOND PAIRS
4
LONE PAIRS
0
H
109.5°
C
BOND ANGLE...
SHAPE...
109.5°
TETRAHEDRAL
H
H
H
METHANE
H
H
H
C
C
H
H
Carbon - has four electrons to pair up
Four covalent bonds are formed
Hydrogen - 1 electron to complete shell
C and H now have complete shells
BOND PAIRS
4
LONE PAIRS
0
BOND ANGLE...
SHAPE...
109.5°
TETRAHEDRAL
AMMONIA
H
N
H
H
N
H
BOND PAIRS
3
LONE PAIRS
1
TOTAL PAIRS
4
• Nitrogen has five electrons in its outer shell
• It cannot pair up all five - it is restricted to eight electrons in its outer shell
• It pairs up only three of its five electrons
• 3 covalent bonds are formed and a pair of non-bonded electrons is left
• As the total number of electron pairs is 4, the shape is BASED on a tetrahedron
AMMONIA
H
H
N
H
H
N
BOND PAIRS
3
LONE PAIRS
1
TOTAL PAIRS
4
• The shape is based on a tetrahedron but not all the repulsions are the same
• LP-BP REPULSIONS > BP-BP REPULSIONS
• The N-H bonds are pushed closer together
• Lone pairs are not included in the shape
N
H
H
N
N
H
H
H
H
H
107°
H
H
ANGLE... 107°
SHAPE... PYRAMIDAL
AMMONIA
H
N
H
H
N
H
BOND PAIRS
3
LONE PAIRS
1
TOTAL PAIRS
4
Spot the errors


Read this set of instructions and write it out again
correcting any mistakes as you go
To predict the shape of different molecules:
A.
B.
C.
D.
E.
Draw a dot and cross diagram for the molecule
Count the total number of electrons around the central
atom (include electrons in the lone pairs)
Decide on the starting shape based on the total number
of lone pairs (eg. 3 lone pairs = trigonal planar)
If lone pairs are present just ignore them
Decide on the name