Two elements ' X ' and 'Y ' have the following configuration

$\begin{array}{rl} & \mathrm{X}=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}4{\mathrm{s}}^2\\ & \mathrm{Y}=1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^5\end{array}$

The compound formed by the combination of ' $\mathrm{X}$ ' and ' $\mathrm{Y}$ ' will be

Element "X" has two extra electrons in its outermost subshell and element "Y" has 1 electron less in its outermost subshell to complete their respective octaves or valency.

Hence, two atoms of Y combine with one atom of X to form a stable compound.so the compound is XY₂