Final answer:
The higher melting point of Mg compared to Na is due to the higher lattice energy resulting from the higher charge density of Mg²+ ions. The covalent character of the Mg-Cl bond also contributes to the difference in melting points.
Step-by-step explanation:
The difference in melting points between NaCl and MgCl₂ can be explained by the difference in their lattice energies. The higher charge density of Mg²+ ions in MgCl₂ compared to Na+ ions in NaCl results in stronger ionic bonds and a higher lattice energy, leading to a higher melting point. However, the observed melting point difference is not solely due to the ionic nature of the compounds.
The Na-Cl bond has a higher percent ionic character (71%) compared to the Mg-Cl bond (58%), resulting in a higher covalent character for the latter. The covalent character reduces the melting point of MgCl₂, despite Mg²+ having a higher charge-to-size ratio than Na+.