The Shapes of Simple Molecules & Ions (OCR A Level Chemistry A): Revision Note
Shapes of Molecules
Molecules can adapt the following shapes and bond angles:
Molecules of different shapes can adapt with their corresponding bond angles
Examples
Examples of molecules with different shapes and bond angles
Worked Example
Draw the shape of the following molecules:
Phosphorus(V) chloride
N(CH3)3
CCl4
Answer 1:
Phosphorus is in group 15, so has 5 valence electrons; Cl is in group 17, so has 17 valence electrons
All 5 electrons are used to form covalent bonds with Cl and there are no lone pairs
This gives a trigonal (or triangular) bipyramidal shape:
Phosphorus pentachloride or phosphorus (V) chloride
Answer 2:
Nitrogen is in group 15, so has 5 valence electrons; carbon is in group 14, so has 4 valence electrons, 3 of which are already used in the covalent bonds with hydrogen
Three of the valence electrons in N are used to form bonding pairs, so there is one lone pair left
N(CH3)3 has a triangular pyramid shape:
Trimethylamine
Answer 3:
Carbon is in group 14, so has 4 valence electrons; chlorine is in group 17, so has 7 valence electrons
All four valence electrons are used to bond with chlorine and there are no lone pairs
The shape of CCl4 is tetrahedral
Tetrachloromethane
Electron Pair Repulsion
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 pairs 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 the same charge
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
Different types of electron pairs have different repulsive forces
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?