Final answer:
AlCl3, with its trivalent cation, has the highest lattice energy and thus is predicted to have the highest melting point among the given compounds.
Step-by-step explanation:
To determine which ionic compound has the highest melting point, we need to consider their lattice energies. Lattice energy is influenced by two major factors: the charge of the ions and the size (or internuclear distance) of the ions. The higher the charge and the smaller the ion, the greater the lattice energy, which typically correlates with a higher melting point. Given the choices BaCl2, AlCl3, CaS, BaS, and CaCl2, we know that a compound with higher charged ions will have a higher lattice energy, and thus a higher melting point.
The lattice energies of ionic compounds also increase with the charge density of the ions. So, comparing these compounds, AlCl3 has a trivalent cation, which would lead to a higher lattice energy compared to the divalent cations in BaCl2, CaS, BaS, and CaCl2. Therefore, based on the charge of the aluminum ion in AlCl3, one can predict it to have the highest melting point among the given compounds.