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Both the ionic solids NaF and MgO have the same number of electrons and about the same inter nuclear distances. But the melting point of NaF is 992°C and that of MgO is 2642°C. Give plausible reason for this observation. Hint:- charge of ions and lattice enthalpy.

Both the ionic solids NaF and MgO have the same number of electrons and about the same inter nuclear distances. But the melting point of NaF is 992°C and that of MgO is 2642°C. Give plausible reason for this observation. Hint:- charge of ions and lattice enthalpy.

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2 Answers

Swapnil Saxena
102 Points
12 years ago

This can be accounted to smaller cation smaller anion stability effect.

Acc. to this rule, only a smaller cation can form a stable salt with smaller anion.

Since F and O r both small anions only a small cation can form stabilised lattices wth them.

Since in this case Na+ is bigger in size than Mg2+, NaF  has a smaller latiice enthalpy than MgO, which has a smaller cation compared to NaF.   

shubhodeep karmakar
13 Points
7 years ago
The crystals of NaF and MgO are formed by Na+ and P in NaF and Mg2+ and O2–ions in MgO respectively arranged in cubic closed structures. There are strong electrovalent bond forces (strong coulombic forces, attraction between ions) between Na+and F– in NaF and Mg2+ and O2– in MgO. But the magnitude of these coulombic forces of attraction is much higher in MgO as compared to that in NaF. (Mg2+ is divalent while Na+is monovalent, similarly O2– and F–). The electrostatic forces of attraction between Mg2+and O2– is almost 4 times as compared to Na+ and F– ions. Melting point of ionic solids is almost the index of inter-ionic attraction in crystal lattice as lot of energy is required to break these forces/overcome these forces before the substance melts.
 

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