20.3    Ion Polarisation

Polarising power and polarisability

• Not all ionic compounds are fully ionic, the same goes to covalent compounds. Pure ionic and covalent bondings are extremes

• In an ionic compound, the positive charge on the cation may attract the negative electron cloud of the anion towards it. This results in distortion of the electron cloud of the anion

• The factors which affect ion polarisation are:
  1. the charge density of the cation
  2. the size of the anion

• A cation is said to have a high charge density if it has a hogh charge spread over a small For example, Mg²⁺ has a higher charge density than Ba²⁺.
  • The higher the charge density of the cation, the higher the polarising  power.
    (Easier to polarise)
  • The larger the size of the anion, the higher the polarisability.
    (Easier to be polarised)
• Ion polarisation causes ionic bonds to have a certain degree of covalent character, making them impure

Ion polarisation and thermal  stability

Refer to Chapter 11: Group II, section 11.4.