Polarisation (Edexcel International A Level (IAL) Chemistry): Revision Note

Polarisation

• 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 and irreversible exchange 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

• 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 leading to a degree of covalent character 

Polarisation of anion leading to covalent character 

Cations

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

• The charge density can be calculated as the charge of the cation divided by the surface area, 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 the anion depends of its ionic radius

• The larger the ionic radius the more easily it will be distorted, for example

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

• A rough approximation of the charge density can be determined by dividing the charge by the square of its ionic radius and this can help us estimate the extent of covalent character

$\mathrm{charge}\mathrm{density}~\frac{\mathrm{charge}}{{\mathrm{r}}^2}$

Comparing the Covalent Character of NaCl and MgCl₂

Comparison of NaCl and MgCl₂: charge density of the cation (charge / radius²): NaCl 1/(0.095)² = 111; MgCl₂ 2/(0.060)² = 556. Experimental lattice energy: NaCl -780 kJ mol^-1, MgCl₂ -2526 kJ mol^-1. Theoretical lattice energy: NaCl -770 kJ mol^-1, MgCl₂ -2326 kJ mol^-1. Percentage difference between experimental and theoretical: NaCl 1.28%, MgCl₂ 7.92%. The experimental value is more exothermic than the theoretical value, and the larger the difference the greater the covalent character: very little for NaCl, more for MgCl₂.