1.5.1 Shapes of Simple Molecules & Ions

• 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

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