Which ionic compound among Al2O3, NaCl, AlF3, . MgO would be expected to have the highest lattice energy?

Lattice energy is the measure of energy that is released when ions bind with each other. This is a result of the cohesive force between these ions. The lattice energy is directly proportional to the charges on the ions. The more the charge, the more the lattice energy released. In the question, Al2O3 is the correct answer.

Al2O3 has the highest number of ions and the charges on Aluminium and Oxygen are 3+ and 2- respectively. Since Al2O3 has the highest number of both charges and ions, it would definitely have the highest lattice enthalpy.

The lattice energy is the energy released when the lattice is formed. It can also be the energy required to break the lattice bonds. These can be covalent or ionic bonds. Depending on the forces between them, the different bonds need specific energy.

In the choices given Al2O3 have the highest number of bonds between them required to create the strong gest attractions which gives the lattice a higher lattice energy. In comparison the remaining structures don’t have sufficiently strong bonds to have a high lattice energy. This means that a lot of energy is required to break these bonds and melt the lattice.