Shapes of Molecules
•be able, in terms of electron pair
repulsion, to predict the shapes of,
and bond angles in, simple
molecules and ions.
Molecules
• The shape of a molecule is determined by the
no. Of electron groups around the central
atom
‘Groups’ might be a
bonding pair of electrons,
a single, non-bonding
electron, a double pair of
bonding electrons, etc...
3.9 Molecular and ionic shapes
VSEPR theory: Valence Shell Electron Pair Repulsion theory.
States that pairs of electrons arrange themselves around the central atom so that
they are as far apart from each other as possible.
Which of these shapes
contains a central atom
with a lone pair?
http://liakatas.org/chemblog/?page_id=17#Vi
deos
SUPER IMPORTANT SUB-RULE!!!
Any lone pairs of non-bonding electrons will be closer to the nucleus of the central atom
There is no second atom
attracting/sharing the lone pair
This increases the repulsion between the lone pair of electrons on A and any other electron
groups on A (bonding or non-bonding)
As the lone pair – ‘other pair’
repulsion increases, the angle
between these pairs decreases.
The X-A-X angle will be slightly
reduced compared to what we would
expect from the simple geometry of
the shape
VSEPR summary
Place these in order of increasing repulsion:
Lone pair - bonding pair
Bonding pair- bonding pair
Lone pair - Lonepair
Complete worksheet 3.9
**
Linear
AX2
Examples
•BeCl2
•CO2
•Bond Angle 180°
Trigonal Planar
AX3
Example
•BF3
•Bond Angle = 120°
Bent
AX2
Example
•H2O
•Bond Angle = 104.5°
Tetrahedral
AX4
Example
•CH4
•Bond Angle = 109.5°
Trigonal Pyramidal
AX3
Example
•NH3
•Bond Angle = 107°
Trigonal Bipyramidal
AX5
Example
•PCl5
•Bond Angles = 120° and 180°
Working it all out
1. Work out the central atom (the one with the smallest
number in the formula)
2. Count the number of bonding pairs to the central
atom
3. Count the number of lone pairs on the central atom
4. Arrange the atoms so the electrons are as far apart as
possible
5. Lone pairs squash bond angles by around 2.5 degrees
6. Treat multiple bonds as single covalent bonds
Total
Electro
n Pairs
Bonding Pair Geometry
Bonding
Pairs
Lone
Pairs
Molecular Geometry
2
180⁰
3
120⁰
120⁰
4
109.5⁰
107⁰
104.5⁰
5
120⁰ & 90⁰
90° & 117°
90°
180⁰
6
90⁰
88⁰
90⁰