Shapes of Covalent Compounds (Edexcel AS Chemistry): Revision Note

Electron Pair Repulsion & Shape

• 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

  • Double and triple bonds are each treated as one region of electron density

  • 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

Shapes & Angles

• 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