Advanced Higher Chemistry
Unit 1
Shapes of molecules and
polyatomic ions
Electron Repulsion
•
•
•
The shapes of molecules are
determined by the numbers of bonding
and non-bonding (lone) electron pairs.
There is repulsion between the orbitals
as a result of the electrons.
Repulsive effect :
bonded:bonded < bonded:lone < lone:lone
Electron Repulsion
•
•
The shape
adopted by the
molecule is the
shape which has
the minimum
repulsion.
The electron
pairs are as far
apart as possible.
Linear Molecules
•
There are two electron
pairs involved in bonding.
•
The electron pairs lie as
far apart as possible
resulting in a linear
molecule.
Trigonal Planar Molecules
•
•
There are three pairs of electrons
involved in bonding.
Repulsion is minimised by the bonding
pairs lying flat at 120o.
Tetrahedral Molecules
•
•
When there are four bonding pairs of
electrons the shape is tetrahedral.
The bond angle is 109.5o.
Tetrahedral Molecules
•
•
If the lone pair of electrons in ammonia
forms a dative covalent bond, a tetrahedral
ammonium ion (NH4+) is formed.
Water also has a tetrahedral arrangement
of electron pairs but the molecule is not
tetrahedral.
Note the
decreasing
angle L to R
Pyramidal Molecules
•
•
Four filled orbitals,
made up of one lone
pair and three bonded
pairs, results in a
pyramidal shape.
The bonded pair
angle is 107o as the
lone pair pushes the
bonded pairs closer
together.
V-shaped/Bent Molecules
•
•
In a water molecule, there are two
bonded pairs and two lone pairs.
The lone pairs repel the bonded pairs
producing a bent molecule with a bond
angle of 105o.
Trigonal Bipyramidal Molecules
•
:
:
Trigonal bipyramidal
molecules are
formed when there
are five filled
bonding pairs of
electrons.
:
T-shaped molecules
•
T-shaped molecules
are formed when
there are three
bonding pairs and
two lone pairs of
electrons.
Octahedral Molecules
•
•
Six bonding pairs results
in an octahedral
molecule.
If the molecule were a
solid shape it would be
an octahedron.
p32
Square pyramidal molecules
•
Square pyramidal
molecules are
formed when there
are five bonding
pairs and one lone
pair of electrons.
Square planar molecules
•
Square planar
molecules are
formed when there
are four bonding
pairs
and two lone pairs
of electrons.
Exercise

Now try the exercise on page 13 of your
notes.