Which of the following represents the correct decreasing order of lattice energy?

Find out the correct order of ionic character in the following molecules:
$(i)$ $O_2$
$(ii)$ $K_2O$
$(iii)$ $N_2$
$(iv)$ $LiF$

Which of the following has the highest melting point?

Write Lewis symbols for the following atoms and ions:
$S$ and $S^{2-}$; $Al$ and $Al^{3+}$; $H$ and $H^{-}$

Two elements $X$ and $Y$ have the following electron configurations:
$X = 1s^2, 2s^2 2p^6, 3s^2 3p^6, 4s^2$
$Y = 1s^2, 2s^2 2p^6, 3s^2 3p^5$
The compound formed by the combination of $X$ and $Y$ is:

The melting point of $RbBr$ is $682\,^{\circ}C$ while that of $NaF$ is $988\,^{\circ}C$. The principal reason for this fact is