• 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

• 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

VSEPR & shapes of moleculesDraw the shape of the following molecules:

1. Phosphorus(V) chloride
2. N(CH3)3
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 use to bond with chlorine and there are no lone pairs
  • The shape of CCl4?is?tetrahedral

Tetrachloromethane