$(i)\, KCl$ or $MgO$ $(ii) \,LiF$ or $LiBr$ $(iii)\, Mg_3N_2$ or $NaCl$
charge on ion (greater the charge higher is the lattice energy)
size of ion (smaller the size of ion greater is the lattice energy)
Among $KCl$ and $MgO , K$ has charge $+1$ and $Mg$ has charge $+2$. Also, $K ^{+}$ has larger ionic radii than $Mg ^{+2}$. So, both the factors add up and the lattice energy of $MgO$ is higher than $KCl$.
$LiF$ and $LiBr$ have the same cation and same ionic charge. So lattice energy depends upon the size of anion. $F ^{-}$ is smaller than $Br ^{-}$, therefore, lattice energy of $LiF$ is higher than $LiBr$.
$MgF _2$ has a charge $+2$ while $NaCl$ has a charge $+1$. Therefore, lattice energy of $MgF _2$ is higher than $NaCl$
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