WAEC Chemistry 2015 Paper

Concept:Find the valency of each element from its atomic number and electron arrangement, then balance their valencies to write the formula.Explanation:Element $\mathrm{X}$ has atomic number $14$. Its electronic configuration is $2, 8, 4$. So, $\mathrm{X}$ has $4$ valence electrons and its valency is $4$.Element $\mathrm{Y}$ has atomic number $16$. Its electronic configuration is $2, 8, 6$. So, $\mathrm{Y}$ has $6$ valence electrons and its valency is $2$.To satisfy both valencies, one atom of $\mathrm{X}$ combines with two atoms of $\mathrm{Y}$.Hence, the simplest formula is $\mathrm{XY}_2$.Answer:B. $\mathrm{XY}_2$