1.7.5 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 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

Bond length

• The bond length is internuclear distance of two covalently bonded atoms
  • It is the distance from the nucleus of one atom to another atom which forms the covalent bond

• The greater the forces of attraction between electrons and nuclei, the more the atoms are pulled closer to each other
• This decreases the bond length of a molecule and increases the strength of the covalent bond
• Triple bonds are the shortest and strongest covalent bonds due to the large electron density between the nuclei of the two atoms
• This increase the forces of attraction between the electrons and nuclei of the atoms
• As a result of this, the atoms are pulled closer together causing a shorter bond length
• The increased forces of attraction also means that the covalent bond is stronger

Triple bonds are the shortest covalent bonds and therefore the strongest ones

Bond Angle

• Molecules can adapt the following shapes and bond angles:

Molecules of different shapes can adapt with their corresponding bond angles