chemrevise.org
19/08/2013
Shapes of molecules
N Goalby
Chemrevise.org
Shapes of molecules
The shape of simple covalent molecules is determined by the
number of bonding pairs of electrons and the number of
lone pair of electrons.
These are called
lone pairs of
electrons or nonbonding pairs
xx
H
x
N
x
H
These are called
bonding pairs of
electrons
x
H
In determining shape the
electron pairs repel away
from each other, and will
move as far away as
possible.
KEY
1
chemrevise.org
19/08/2013
Linear shape : 2 bonding pairs
Cl
Be
Cl
Bond Angle= 180o
BeCl2 (Be does not have a full outer shell).
It does not agree with the octet rule (more on this later)
Other examples:
CS2, HCN, BeF2
KEY
Trigonal planar: 3 bonding pairs of electrons
Cl
Cl
Cl
Cl
B
B
Bond Angle
= 120o
Cl
Cl
Cl
x
x
Cl
B
x
Cl
BCl3
[B only has 6 electrons in its
outer shell: it also has an
incomplete octet.]
Other examples:
BF3, AlCl3, SO3,
NO3-, CO32-
KEY
2
chemrevise.org
19/08/2013
Tetrahedral: 4 bonding pairs of electrons
H
CH4
Bond Angle
= 109.5o
C
H
All carbon compounds with 4 single
bonds around the carbon will be
tetrahedrally arranged e.g. CF4, CH3Cl
H
H
H
x
H
x
C
x
Silicon, sulphur, and the
halogens can form
tetrahedral species e.g.
SiCl4, SO42-, ClO4-
H
x
H
KEY
Trigonal pyramidal: 3 bonding pairs and 1 lone pair
A lone pair is a pair of electrons not involved in a bond.
They take up volume, however, and therefore affect shape.
Lone pair
xx
N
H
H
H
H
Bond angle =
107o
Lone pairs repel bonded
pairs MORE THAN
bonded pairs repel
bonded pairs
x
N
x
H
x
H
The repulsion of the lone pair of electrons pushes
the N-H bonds closer together than in CH4, so the
H-N-H bond angle is 107o
Other examples of trigonal pyrimidal
NCl3 ,PF3 ,ClO3 ,H3O+
KEY
3
chemrevise.org
19/08/2013
Bent : 2 bonding pairs and 2 lone pairs
H2 O
The two lone pairs of
electrons in water repel
more than the bonding pairs
and so the bond angle is
smaller than in tetrahedral or
trigonal pyrimidal.
O
H
H
Bond angle =
104.5o
Examples:
OCl2, H2S, OF2 , SCl2
KEY
Lone pairs
H
C
H
O
N
H
H
109.5o
H
bonding-pair vs. bonding
pair repulsion
<
107o
H
H
H
lone-pair vs. bonding
pair repulsion
104.5o
<
H
lone-pair vs. lone pair
repulsion
4
chemrevise.org
19/08/2013
Effect of double and single covalent bonds
Double and triple bonds act like single bonds in
determining the shape of the molecule. So CO2 acts like it has
two bonded pairs of electrons
O
C
O
180°
O
C
O
NOTE:
1. The electron pairs on the central atom (in this case, carbon) determine the shape
2. It is more appropriate to explain the shape in terms of ‘number of regions of
negative charge’. CO2 has two regions of negative charge, and has the linear
shape of BeCl2.
KEY
General way to explain shape
1. State number of bonding pairs and lone pairs of electrons.
2. State that electrons repel and try to get as far apart as
possible (or to a position of minimum repulsion.)
3. If there are no lone pairs state that the electron pairs repel
equally
4. If there are lone pairs of electrons, then state that lone pairs
repel more than bonding pairs.
5. State actual shape and bond angle.
It is important to state electron pairs repel and not
atoms/elements/bonds
KEY
5
chemrevise.org
19/08/2013
Expanding octet of Sulphur
How electrons are arranged when S forms two bonds
3p
3s
3d
Sulphur can use 3d orbitals
promotion of two electrons to 3d
How electrons are arranged when S forms six bonds
3s
3p
3d
S can now have more than eight electrons in its outer shell.
So it can form six bonds.
10 electrons in outer shell: 5 bonding pairs
Trigonal Bipyrimidal : PF5
F
90o
F
120o
F
P
F
F
KEY
6
chemrevise.org
19/08/2013
12 electrons in outer shell: 6 bonding pairs
F
SF6 : Octahedral Shape
F
F
All bond angles
are 90o
S
F
F
F
KEY
Electron-group repulsions and the five
basic molecular shapes.
linear
trigonal planar
trigonal bipyrimidal
tetrahedral
Octahedral
KEY
7
chemrevise.org
19/08/2013
Other shapes
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
2
1
Applied
understanding
Arrangement of
electron pairs
Molecular
Geometry
bent
trigonal planar
S
O
O
Example SO2
Other shapes
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
4
1
Applied
understanding
Arrangement of
electron pairs
trigonal
bipyramidal
Molecular
Geometry
distorted
tetrahedron
F
F
S
F
F
Bond angles ~119 + 89O
S has 6 electrons in its outer shell. 4 F’s
add 4 more electrons. This makes a total of
10 electrons made up of 4 bond pairs and
1 lone pairs. The means it is a variation of
the 5 bond pair shape (trigonal pyramidal)
8
chemrevise.org
19/08/2013
Other shapes
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
3
2
Applied
understanding
Arrangement of
electron pairs
Molecular
Geometry
trigonal
bipyramidal
T-shaped
F
Cl
F
F
Cl has 7 electrons in its outer shell. 3F’s
add 3 more electrons. This makes a total of
10 electrons made up of 3 bond pairs and 2
lone pairs. The means it is a variation of
the 5 bond pair shape (trigonal pyramidal)
Bond angle ~89O
Other shapes
Applied
understanding
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
Arrangement of
electron pairs
2
3
trigonal
bipyramidal
Molecular
Geometry
linear
I
I
I
Central I has 7 electrons in its outer shell. 2I’s add 2
more electrons. Negative charge adds extra electron.
This makes a total of 10 electrons made up of 2 bond
pairs and 3 lone pairs. The means it is a variation of
the 5 bond pair shape (trigonal pyramidal)
I3 -
9
chemrevise.org
19/08/2013
Other shapes
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
5
1
Applied
understanding
Arrangement of
electron pairs
Molecular
Geometry
square
pyramidal
octahedral
F
F
F
Br
F
F
Br has 7 electrons in its outer shell. 5F’s add
5 more electrons. This makes a total of 12
electrons made up of 5 bond pairs and 1 lone
pairs. The means it is a variation of the 5
bond pair shape (trigonal pyramidal)
Other shapes
No. of atoms
bonded to
central atom
4
No. lone
pairs on
central atom
2
Applied
understanding
Arrangement of
electron pairs
Molecular
Geometry
octahedral
square
planar
F
F
Xe
F
Bond angle
F
90O
Xe has 8 electrons in its outer shell. 4
F’s add 4 more electrons. This makes a
total of 12 electrons made up of 4 bond
pairs and 2 lone pairs. The means it is a
variation of the 6 bond pair shape
(octahedral)
10
chemrevise.org
19/08/2013
Shapes of
Sulphur
compounds
O
S
HO
O
OH
H2SO4
O
S
-O
O
O
S
O-
SO32-
S
O
SO3
O
O
SO2
11
chemrevise.org
19/08/2013
Shapes of molecules
N Goalby
Chemrevise.org
Shapes of molecules
The shape of simple covalent molecules is determined by the
number of bonding pairs of electrons and the number of
lone pair of electrons.
These are called
lone pairs of
electrons or nonbonding pairs
xx
H
x
N
x
H
These are called
bonding pairs of
electrons
x
H
In determining shape the
electron pairs repel away
from each other, and will
move as far away as
possible.
KEY
1
chemrevise.org
19/08/2013
Linear shape : 2 bonding pairs
Cl
Be
Cl
Bond Angle= 180o
BeCl2 (Be does not have a full outer shell).
It does not agree with the octet rule (more on this later)
Other examples:
CS2, HCN, BeF2
KEY
Trigonal planar: 3 bonding pairs of electrons
Cl
Cl
Cl
Cl
B
B
Bond Angle
= 120o
Cl
Cl
Cl
x
x
Cl
B
x
Cl
BCl3
[B only has 6 electrons in its
outer shell: it also has an
incomplete octet.]
Other examples:
BF3, AlCl3, SO3,
NO3-, CO32-
KEY
2
chemrevise.org
19/08/2013
Tetrahedral: 4 bonding pairs of electrons
H
CH4
Bond Angle
= 109.5o
C
H
All carbon compounds with 4 single
bonds around the carbon will be
tetrahedrally arranged e.g. CF4, CH3Cl
H
H
H
x
H
x
C
x
Silicon, sulphur, and the
halogens can form
tetrahedral species e.g.
SiCl4, SO42-, ClO4-
H
x
H
KEY
Trigonal pyramidal: 3 bonding pairs and 1 lone pair
A lone pair is a pair of electrons not involved in a bond.
They take up volume, however, and therefore affect shape.
Lone pair
xx
N
H
H
H
H
Bond angle =
107o
Lone pairs repel bonded
pairs MORE THAN
bonded pairs repel
bonded pairs
x
N
x
H
x
H
The repulsion of the lone pair of electrons pushes
the N-H bonds closer together than in CH4, so the
H-N-H bond angle is 107o
Other examples of trigonal pyrimidal
NCl3 ,PF3 ,ClO3 ,H3O+
KEY
3
chemrevise.org
19/08/2013
Bent : 2 bonding pairs and 2 lone pairs
H2 O
The two lone pairs of
electrons in water repel
more than the bonding pairs
and so the bond angle is
smaller than in tetrahedral or
trigonal pyrimidal.
O
H
H
Bond angle =
104.5o
Examples:
OCl2, H2S, OF2 , SCl2
KEY
Lone pairs
H
C
H
O
N
H
H
109.5o
H
bonding-pair vs. bonding
pair repulsion
<
107o
H
H
H
lone-pair vs. bonding
pair repulsion
104.5o
<
H
lone-pair vs. lone pair
repulsion
4
chemrevise.org
19/08/2013
Effect of double and single covalent bonds
Double and triple bonds act like single bonds in
determining the shape of the molecule. So CO2 acts like it has
two bonded pairs of electrons
O
C
O
180°
O
C
O
NOTE:
1. The electron pairs on the central atom (in this case, carbon) determine the shape
2. It is more appropriate to explain the shape in terms of ‘number of regions of
negative charge’. CO2 has two regions of negative charge, and has the linear
shape of BeCl2.
KEY
General way to explain shape
1. State number of bonding pairs and lone pairs of electrons.
2. State that electrons repel and try to get as far apart as
possible (or to a position of minimum repulsion.)
3. If there are no lone pairs state that the electron pairs repel
equally
4. If there are lone pairs of electrons, then state that lone pairs
repel more than bonding pairs.
5. State actual shape and bond angle.
It is important to state electron pairs repel and not
atoms/elements/bonds
KEY
5
chemrevise.org
19/08/2013
Expanding octet of Sulphur
How electrons are arranged when S forms two bonds
3p
3s
3d
Sulphur can use 3d orbitals
promotion of two electrons to 3d
How electrons are arranged when S forms six bonds
3s
3p
3d
S can now have more than eight electrons in its outer shell.
So it can form six bonds.
10 electrons in outer shell: 5 bonding pairs
Trigonal Bipyrimidal : PF5
F
90o
F
120o
F
P
F
F
KEY
6
chemrevise.org
19/08/2013
12 electrons in outer shell: 6 bonding pairs
F
SF6 : Octahedral Shape
F
F
All bond angles
are 90o
S
F
F
F
KEY
Electron-group repulsions and the five
basic molecular shapes.
linear
trigonal planar
trigonal bipyrimidal
tetrahedral
Octahedral
KEY
7
chemrevise.org
19/08/2013
Other shapes
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
2
1
Applied
understanding
Arrangement of
electron pairs
Molecular
Geometry
bent
trigonal planar
S
O
O
Example SO2
Other shapes
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
4
1
Applied
understanding
Arrangement of
electron pairs
trigonal
bipyramidal
Molecular
Geometry
distorted
tetrahedron
F
F
S
F
F
Bond angles ~119 + 89O
S has 6 electrons in its outer shell. 4 F’s
add 4 more electrons. This makes a total of
10 electrons made up of 4 bond pairs and
1 lone pairs. The means it is a variation of
the 5 bond pair shape (trigonal pyramidal)
8
chemrevise.org
19/08/2013
Other shapes
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
3
2
Applied
understanding
Arrangement of
electron pairs
Molecular
Geometry
trigonal
bipyramidal
T-shaped
F
Cl
F
F
Cl has 7 electrons in its outer shell. 3F’s
add 3 more electrons. This makes a total of
10 electrons made up of 3 bond pairs and 2
lone pairs. The means it is a variation of
the 5 bond pair shape (trigonal pyramidal)
Bond angle ~89O
Other shapes
Applied
understanding
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
Arrangement of
electron pairs
2
3
trigonal
bipyramidal
Molecular
Geometry
linear
I
I
I
Central I has 7 electrons in its outer shell. 2I’s add 2
more electrons. Negative charge adds extra electron.
This makes a total of 10 electrons made up of 2 bond
pairs and 3 lone pairs. The means it is a variation of
the 5 bond pair shape (trigonal pyramidal)
I3 -
9
chemrevise.org
19/08/2013
Other shapes
No. of atoms
bonded to
central atom
No. lone
pairs on
central atom
5
1
Applied
understanding
Arrangement of
electron pairs
Molecular
Geometry
square
pyramidal
octahedral
F
F
F
Br
F
F
Br has 7 electrons in its outer shell. 5F’s add
5 more electrons. This makes a total of 12
electrons made up of 5 bond pairs and 1 lone
pairs. The means it is a variation of the 5
bond pair shape (trigonal pyramidal)
Other shapes
No. of atoms
bonded to
central atom
4
No. lone
pairs on
central atom
2
Applied
understanding
Arrangement of
electron pairs
Molecular
Geometry
octahedral
square
planar
F
F
Xe
F
Bond angle
F
90O
Xe has 8 electrons in its outer shell. 4
F’s add 4 more electrons. This makes a
total of 12 electrons made up of 4 bond
pairs and 2 lone pairs. The means it is a
variation of the 6 bond pair shape
(octahedral)
10
chemrevise.org
19/08/2013
Shapes of
Sulphur
compounds
O
S
HO
O
OH
H2SO4
O
S
-O
O
O
S
O-
SO32-
S
O
SO3
O
O
SO2
11