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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.

amrith h , 12 Years ago

Grade

2 Answers

Swapnil Saxena

Last Activity: 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

Last Activity: 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.