Final answer:
Ionic compounds are ranked by increasing lattice energy and melting point, considering the charges and sizes of their ions. NaCl has lower lattice energy compared to CaO and MgO because it contains ions with lower charges. MgO has the highest lattice energy among them due to the small size of Mg2+ and high charge of O2-.
Step-by-step explanation:
The goal is to rank the ionic compounds NaCl, MgO, and CaO according to increasing lattice energy and melting point. To do this, we must consider the charges on the ions and the ionic radii. Lattice energy increases with the charge of the ions and decreases with increasing ionic size because the internuclear distance increases. NaCl consists of Na+ and Cl-, MgO contains Mg2+ and O2-, and CaO contains Ca2+ and O2-. Because O2- has a higher charge than Cl-, the compounds containing O2- will have higher lattice energies than NaCl. Between CaO and MgO, we consider ionic radii; because Mg2+ is smaller than Ca2+, it will have a stronger attraction to O2-, leading to a higher lattice energy for MgO compared to CaO. Therefore, the correct order from lowest to highest lattice energy and melting point is NaCl < CaO < MgO.