1. Ionic Bonding Strength:
* MgO: Magnesium and oxygen have a large difference in electronegativity, leading to a very strong ionic bond. The small ionic radii of Mg²⁺ and O²⁻ further enhance the electrostatic attraction between them.
* NaCl: Sodium and chlorine have a smaller electronegativity difference, resulting in a weaker ionic bond. Sodium and chloride ions are also larger, which further reduces the electrostatic attraction between them.
2. Lattice Structure:
* MgO: MgO adopts the rock salt structure, a highly symmetrical and stable lattice structure. The close packing of ions in this structure contributes to its high melting point.
* NaCl: NaCl also adopts the rock salt structure, but the weaker ionic bonds result in a less stable lattice compared to MgO.
3. Covalent Character:
* MgO: Despite being primarily ionic, MgO exhibits some degree of covalent character due to the small size and high charge density of the ions. This covalent character further strengthens the bond.
* NaCl: NaCl has a much lower degree of covalent character, contributing to its weaker bond.
4. Polarizability:
* MgO: Magnesium and oxygen ions are less polarizable than sodium and chloride ions. This means they are less likely to distort their electron clouds under the influence of neighboring ions, leading to a more stable lattice.
5. Van der Waals forces:
* MgO: The weak van der Waals forces between MgO molecules are negligible compared to the strong ionic bond.
* NaCl: While the ionic bond in NaCl is weaker than in MgO, it still plays a significant role in holding the lattice together. However, van der Waals forces between NaCl molecules are stronger due to their larger size and greater polarizability.
In summary, the significantly stronger ionic bond, more stable lattice structure, and lower polarizability of MgO contribute to its much higher melting point compared to sodium chloride.
