Elements X and Y have valence shell electronic configuration as, X, ns² np⁵ ; Y : ns¹
Which compound is likely to be formed from X and Y ?

$\begin{array}{c}\mathrm{X}:{\mathrm{ns}}^2{\mathrm{np}}^5\text{ (electronegative element) }\\ \text{ Valency }=-1\\ \mathrm{Y}:{\mathrm{ns}}^1\text{ (electropositive alkali metal) }\\ \text{ Valency }=+1\end{array}$
$\left.\begin{array}{l}\mathrm{Y}⟶{\mathrm{Y}}^{+}+{\mathrm{e}}^{-}{\mathrm{Y}}^{+}\\ \mathrm{X}+{\mathrm{e}}^{-}⟶{\mathrm{X}}^{-}{\mathrm{X}}^{-}\end{array}\right]\mathrm{have} \mathrm{inert} \mathrm{gas} \mathrm{configuration}$
Thus, compound is YX.