CIE A Level Chemistry复习笔记1.3.11 Shapes of Molecules

• 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

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

Worked example: VSEPR & shapes of molecules

Answer 1

Answer 2

Answer 3