Polarisation (Edexcel International AS Chemistry): Revision Note

The Polarisation of anions & Cations

• Theoretical lattice energies assume a perfect ionic model where the ions are 100% spherical and the attractions are purely electrostatic 

  • So overall, there has been a complete transfer of outer shell electrons between the two elements in the compound so that one ion is negatively charged and the other ion positively charged and the charges are whole values

  • Comparing theoretical and experimental lattice energies is covered at A level, in Unit 4

• Theoretical lattice energy can differ from measured lattice energy

• This is because covalent character is introduced when there is polarisation of the anions

• When this occurs, the cation attracts electrons from the anion therefore distorting electron density of the anion 

Polarisation of anion leading to covalent character 

Cations

• A cation with a large charge density will have a larger polarising power 

• The charge density depends on the charge of the cation and its ionic radius: the smaller and more highly charged the cation, the higher its charge density, for example

  • K⁺ will have a lower polarising power than Li⁺ as the ionic radius is greater

  • Na⁺ will have a lower polarising power than Mg²⁺ as the charge is smaller 

• So a cation that is small and highly charged will have the greatest polarising power

Anions 

• Anions are polarised

• The polarisability of an anion depends on its ionic radius and its charge

• The larger the ionic radius, or the greater the charge, the more easily an anion is distorted, for example

  • Br- will be more polarisable than Cl^- as the ionic radius is larger