The lattice energy of a salt is related to the energy required to separate the ions. For which of the following pairs of ions is the energy that is required to separate the ions largest? (Assume that the distance between the ions in each pair is equal to the sum of the ionic radii.)

Answer: The Lattice energy is the energy required to separate an ionic solid into its component gaseous ions or

It is the energy released when gaseous ions combine to form an ionic solid.

Explanation:

The lattice energy depends on the ionization energies and electron affinities of atoms involved in the formation of the compound. The ionization energies and electron affinities also depends on the ionic radius and charges of the ions involved. As the ionic radius for cations increases down the groups, ionization energy decreases, whereas, as ionic radii decreases across the periods, ionization energy increases. The trend observed for anions is that as ionic radii increase down the groups, electron affinity decreases. Across the period, as ionic radii increases electron affinity increases. Also, as the charge on the ion increases, it leads to an increase in energy requirement/content.

Therefore, for compounds formed from cations and anions in the same period, the highest charged cation and anion will have the highest lattice energy. For example, among the following compounds: Al2O3 (aluminium oxide), AlCl3 (aluminium chloride), MgO, MgCl2 (magnesium chloride), NaCl, Na2O (sodium oxide); Al2O3 (aluminium oxide) will have the highest lattice energy, thus will be hardest to break apart because its ions have the highest charge.