Magnesium oxide (MgO) has a higher boiling and melting point than sodium chloride (NaCl) due to the differences in the type and strength of the ionic bonds present in these compounds.
Firstly, MgO consists of magnesium ions (Mg2+) and oxide ions (O2-). The charges on these ions are higher compared to those in NaCl, which consists of sodium ions (Na+) and chloride ions (Cl–). The presence of the divalent Mg2+ and O2- ions in MgO leads to stronger electrostatic forces between the ions because the greater the charge, the stronger the ionic bond.
Secondly, the lattice structure of MgO is more compact and requires more energy to break apart compared to that of NaCl. MgO forms a face-centered cubic crystal lattice that is more stable due to this compact arrangement, which contributes to its high melting and boiling points.
In contrast, NaCl’s ions have only single charges, leading to weaker ionic bonds and a less stable lattice structure. As a result, NaCl has lower melting and boiling points than MgO.