Shapes of Molecules (Cambridge (CIE) AS Chemistry)
Revision Note
VSEPR Theory
The valence shell electron pair repulsion theory (VSEPR) predicts the shape and bond angles of molecules
Electrons are negatively charged and will repel other electrons when close to each other
In a molecule, the bonding pair of electrons will repel other electrons around the central atom forcing the molecule to adopt a shape in which these repulsive forces are minimised
When determining the shape and bond angles of a molecule, the following VSEPR rules should be considered:
Valence shell electrons are those electrons that are found in the outer shell
Electron pairs repel each other as they have similar charges
Lone pair electrons repel each other more than bonded pairs
Repulsion between multiple and single bonds is treated the same as for repulsion between single bonds
Repulsion between pairs of double bonds are greater
The most stable shape is adopted to minimize the repulsion forces
Different types of electron pairs have different repulsive forces
Lone pairs of electrons have a more concentrated electron charge cloud than bonding pairs of electrons
The cloud charges are wider and closer to the central atom’s nucleus
The order of repulsion is therefore: lone pair – lone pair > lone pair – bond pair > bond pair – bond pair
Repulsion between different types of electron pairs
Different types of electron pairs have different repulsive forces
Molecules can adapt the following shapes and bond angles:
Bond shapes and bond angles
Molecules of different shapes can adapt with their corresponding bond angles
Examples of molecules with different shapes and bond angles
Each different shape has a specific name and specific bond angle(s)
Worked Example
VSEPR & shapes of molecules
Draw the shape of the following molecules and compounds:
Phosphorous(V) chloride
N(CH3)3
CCl4
Answer 1
Phosphorous has a +5 charge so 5 Cl– ions are needed to neutralise the charge
Therefore, the phosphorous(V) chloride is PCl5
P is in Group 5 and has 5 valence / outer electrons
Cl is in Group 7 (17) and has 7 valence / outer electrons
All 5 electrons of phosphorous are used to form single covalent bonds to the 5 chlorines
There are no lone pairs
So, phosphorous(V) chloride has a trigonal bipyramidal shape
Answer 2
Nitrogen is in Group 5 and has 5 valence / outer electrons
The carbon in the -CH3 groups is in Group 4 and has 4 valence / outer electrons
3 of these electrons are already used in covalent bonds with hydrogen
3 of N's valence / outer electrons are involved in bonding pairs with the carbon from the -CH3 groups
This leaves one pair of electrons as a lone pair
So, N(CH3)3 has a pyramidal shape
Answer 3
Carbon is in Group 4 and has 4 valence / outer electrons
Cl is in Group 7 (17) and has 7 valence / outer electrons
All 4 valence / outer electrons of carbon are used as bonding pairs with the 4 chlorines
There are no lone pairs
So, the shape of CCl4 is tetrahedral
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