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
