Which of the following compounds is ionic?

The lattice energies of $KF$,$KCl$,$KBr$ and $KI$ follow the order:

The atomic numbers of the elements $X, Y, Z$ are $a, a+1, a+2$ respectively. $Z$ is an alkali metal. The nature of bonding in the compound formed by $X$ and $Z$ is

Which ionic compound has the largest amount of lattice energy?

Use Lewis symbols to show electron transfer between the following atoms to form cations and anions:
$(a)$ $K$ and $S$
$(b)$ $Ca$ and $O$
$(c)$ $Al$ and $N$

Which of the following substances has the highest melting point?