Final answer:
The melting points of NaCl and MgCl₂ are high due to strong ion-ion bonding, but the covalent character of the bonds in MgCl₂ reduces its melting point despite having a higher charge-to-size ratio than NaCl.
Step-by-step explanation:
Both NaCl and MgCl₂ are ionic solids with strong ion-ion bonding, which is why they have high melting temperatures. The lattice energies of NaCl and MgCl₂ are 876 kJ/mol and 2526 kJ/mol, respectively, indicating strong ionic bonds. However, the observed melting points cannot be fully explained by solely considering ionic bonds. The Na-Cl bond has 71% ionic character, while the Mg-Cl bond has 58% ionic character. The covalent character of the bonds reduces the melting point of MgCl₂, even though Mg²+ has a higher charge-to-size ratio than Na+.