Shapes & Angles
- 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
