Why is the melting point of NaCl higher than that of MgCl2?
Sodium chloride (NaCl) has a melting point of 801 degree Celsius, while that of magnesium chloride (MgCl2) is 714 degrees Celsius. Both sodium chloride and magnesium chloride are ionic solids. Such compounds (that contain ionic bonds) have a very high degree of attraction between bonding atoms, which results in high melting points. The high melting points are evident (801 and 714 degree Celsius). Even though it would appear that magnesium chloride should have a higher melting point given the two chloride ions (and the resultant high attractive forces), yet sodium chloride has a higher melting point. This is because the bonds that exist between sodium and chloride, and magnesium and chloride are not entirely ionic and some degree of covalency exists. In fact, magnesium chloride has a greater degree of covalency as compared to sodium chloride. Also, magnesium is more electronegative as compared to sodium. These factors combine to result in a higher force of attraction in sodium chloride as compared to magnesium chloride and hence a higher melting point for the former.
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