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Both the cations, Li+ and Mg2+ are of comparable size. In LiF we have a 1:1 charge distribution between the ions whereas, in MgO it is 2:2. Just by comparing the strength of the ionic bond which is proportional to the product of the two charges, it is evidently four times for MgO than in LiF. Consequently, the strengths of ionic bonds in crystals of magnesium oxide will be higher than lithium fluoride. This huge difference in strength is further corroborated from the large difference in observed melting point values of the two ionic compounds; MgO – 2852ᵒC, LiF – 845ᵒC and also by comparing their lattice energy values: MgO –(-3795)kJ/mol, LiF –(-1030) kJ/mol.
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