Which substance do you expect to have the greatest lattice energy: MgF2, CaF2, or ZrO2? Why
The Correct Answer and Explanation is :
Among the given substances, ZrO₂ (zirconium dioxide) is expected to have the greatest lattice energy. This is due to the combination of the following factors:
1. Charge of Ions
Lattice energy increases with the magnitude of ionic charges. ZrO₂ consists of Zr⁴⁺ cations and O²⁻ anions, which have higher charges compared to the Mg²⁺ and Ca²⁺ cations in MgF₂ and CaF₂, and the F⁻ anions in these compounds. The greater charges in ZrO₂ create a stronger electrostatic attraction, significantly increasing the lattice energy.
2. Ionic Radii
Lattice energy is inversely proportional to the sum of the ionic radii of the interacting ions. Zirconium ions (Zr⁴⁺) are smaller compared to Mg²⁺ or Ca²⁺ due to the high charge-to-radius ratio, which results in stronger Coulombic forces. Additionally, oxygen ions (O²⁻) are slightly larger than fluoride ions (F⁻), but the overall effect of the smaller Zr⁴⁺ radius dominates.
3. Bonding Characteristics
While MgF₂ and CaF₂ are largely ionic, ZrO₂ has partially covalent characteristics due to the polarizability of the oxygen ion and the high charge density of Zr⁴⁺. This enhances the overall lattice energy by increasing bond strength.
Conclusion
The lattice energy of ionic compounds depends on the charges of ions and their sizes. ZrO₂ has ions with higher charges (Zr⁴⁺ and O²⁻) compared to MgF₂ (Mg²⁺ and F⁻) and CaF₂ (Ca²⁺ and F⁻). Additionally, the smaller ionic radius of Zr⁴⁺ contributes to a stronger electrostatic attraction, resulting in the greatest lattice energy for ZrO₂. The greater ionic charge and reduced ionic size in ZrO₂ lead to significantly stronger interactions, making its lattice energy the highest among the three compounds.